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Cavolinia tridentata (Niebuhr, 1775) forma occidentalis Dall, 1908b


This is a thecosomatous pteropod, 1.1 cm long. This is the smallest forma in this species. Dorsal surface is strongly vaulted with three moderately developed central and two faintly developed lateral dorsal ribs. The caudal spine is straight and relatively long. Upper aperture lip rounded, no keel along the lateral sides. Ventral side is strongly vaulted in profile and perfectly rounded. Aperture triangular lunar-shaped. The sculpture consists of growth lines, up to 20 transverse striae ventrally. Micro-zooplankton and phytoplankton are its food and it is a mucus feeder. It lives in the NE-Pacific Ocean, at moderate depths (Cavolinia t. occidentalis dorsal).

Taxonomic Description

The shell is relatively small compared to the other formae of this species. In general appearance the specimens resemble Cavolinia uncinata, though the caudal spine is completely straight. The opaque shell shows a relatively strongly vaulted dorsal surface with three moderately developed central ribs and two faintly developed lateral ribs. The dorsal aperture lip is separated from the dorsal surface by a prominent arch-shaped rib. The dorsal lip is rounded and the lateral sides have no keel. The tops of the lateral spines are not or only slightly directed caudad. The shell margin between the caudal spine and lateral spines is composed of a more or less straight section and a concave portion. The ventral aperture lip is small in comparison with the other forms. The ventral side is strongly vaulted, in profile it is perfectly round. The aperture has a triangular lunar shape (Cavolinia t. occidentalis drawing, Cavolinia t. occidentalis, Cavolinia t. occidentalis lateral). The shell sculpture consists of growth lines and up to 20 ventral transverse ribs. The shell shows only a light brown or yellowish hue, the upper lip and lateral sides are lighter.
The shell length varies between 7.5 and 11.5 mm, the width between 5.5 and 8.5 mm, with averages of 10.3 and 6.6 mm respectively.


A special description is not available.


This form is protandric hermaphrodite.


This form is phytophagous.


This forma is restricted to the NE-Pacific, see the Cavolinia t. occidentalis map.


Cavolina occidentalis Dall, 1908: 223, pl. 12, fig. 1.
The holotype (USNM. 110591) and four paratypes (USNM. 110590) are kept dry in the USNM.
Type locality: 38°4'N 137°W, coll. Dall.

Cavolinia tridentata occidentalis