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Author: Gill, 1864

Field Marks:
A spotless Mustelus with short head, broad internarial space, relatively small eye, narrow head, short mouth, upper labial furrows about equal to lowers, lanceolate lateral denticles with ridges incomplete when present, triangular dorsal fins, with the first dorsal closer-to the pelvics than the pectorals, a poorly developed ventral caudal lobe, and 89 to 98 precaudal vertebral centra.

Diagnostic Features:
Body fairly slender. Head short, prepectoral length 16 to 20% of total length; snout moderately long and bluntly angular in lateral view, preoral snout 5 to 7.2% of total length, preorbital snout 6.1 to 8% of total length; internarial space broad, 2.1 to 2.9% of total length; eyes fairly small, eye length 2.5 to 3.4 times in preorbital snout and 1.9 to 3.3% of total length; interorbital space narrow, 3.5 to 4.5% of total length; mouth short, subequal to eye length, its length 2.3 to 3.3% of total length; upper labial furrows usually equal to lowers, upper furrows 0.9 to 1.8% of total length; teeth molariform and asymmetric, with cusp reduced to a low point, cusplets absent; condition of buccopharyngeal denticles unknown. Interdorsal space 17 to 21% of total length; trailing edges of dorsal fins denticulate, without bare ceratotrichia; first dorsal broadly triangular to semifalcate with posteroventrally sloping or nearly vertical posterior margin, its midbase closer to pelvic bases than to pectorals; pectoral fins moderatesized, length of their anterior margins 12 to 15% of total length, width of posterior margins 7.4 to 12% of total length; pelvic fins moderate-sized, length of anterior margins 7.1 to 9.2% of total length; anal height 2.3 to 3.7% of total length; anal-caudal space greater than second dorsal height, 5.9 to 8.6% of total length; ventral caudal lobe notfalcate or somewhat falcate in adults. Crowns of lateral trunk denticles lanceolate, longitudinal ridges absent or extending about half of their entire length. Skeleton not hypercalcified in adults; palatine processes of palatoquadrates subdivided at symphysis, with a short separate medial segment on each side; monospondylous precaudal centra 32 to 40, diplospondylous precaudal centra 52 to 61, precaudal centra 89 to 98. Colour uniform grey above, light below, no white or dark spots or dark bars. Development viviparous. Size large, adults 57 to 124 cm.

Geographical Distribution:
Eastern North Pacific: Northern California to Gulf of California.

Habitat and Biology:
A common inshore and offshore warm-temperate to tropical bottom-dwelling shark of the continental shelves, entering shallow muddy bays. In north-central Californian waters in the USA it is primarily a summer visitor, but is resident in warmer waters from southern California south.

Viviparous, with a yolk-sac placenta; number of young 2 to 5 per litter.

Feeds mostly on crabs, including cancrids and grapsids, with the smaller grapsids being more important to younger sharks than larger ones, which eat more cancrids; also ghost shrimp, innkeeper (echiuroid) worms (Urechis), and small fish (herring and midshipmen, Porichthys) are occasionally taken. Burrowing invertebrates such as ghost shrimp and innkeeper worms are uncommon prey, indicating that this shark probably seldom roots out or sucks out such food, and may take only the stray individuals washed out of their burrows by the tide.

Size:
Maximum about 124 cm, males maturing between 57 and 65 cm and reaching 116 cm; females maturing about 70 cm and reaching at least 124 cm.

Interestto Fisheries:
Regularly caught by longline fisheries in the Gulf of California and utilized for human consumption; also caught by anglers in California.

Remarks:
According to Heemstra (1973), the holotype of this species was apparently lost. The holotype was said to have been collected at San Francisco, California, USA. As this species is not resident in the San Francisco area but is a summer visitor to the Monterey area, some 160 km south, Heemstra (1973) doubted the type locality of this species, and suggested that the holotype was collected in southern California. Another possibility suggests itself, that the specimen actually was taken at San Francisco, but was further north than the usual range of the species because of a warm water mass ('el Nino') moving up the coast.

In the southern part of its range this species is sympatric with M. lunulatus, but the latter can be distinguished by a broader interorbital, shorter mouth, often more falcate fins, shorter upper labial furrows, and fewer precaudal centra.

Type material:
Holotype: ?. Type Locality: San Francisco, California.

Grey smooth-hound (Mustelus californicus)