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n.1.(Zool.) The embryonic shell, or first chamber, of ammonites and other cephalopods.
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Diagnosis: Tall, high spired with large protoconch, almost straight whorls, and, strong axial ribs crossed by microstriae.
elevatus the maximum distance from the edge of the shell to the edge of the protoconch was used, as total length is not found to be a sensitive indicator of growth in juveniles (Sadosky et al.
Sirenko (1993) noted the large protoconch (larval shell) and pelagic larva of Fusitriton oregonensis, which has a reported range from California to northern Japan (Beu, 1978) that includes the isolated seamounts Cobb and Patton (Birkeland, 1971; Somerton, 1981).
In the Caecinae, shells have deciduous smooth, flattened, spiral protoconch that is lost during the juvenile life and replaced by a septum.
The protoconch is present, but does not entirely cover the larva (VIII-P).
The absence of a protoconch 2, which is formed during a feeding veliger stage, suggests that these limpets possess lecithotrophic veligers, which in turn suggests they their dispersal capability might be limited (Lutz et al.
Differs from the three preceding species of Cerastidae by the wider bulimoid shell with a cylindrical umbilicus, and by the characteristic protoconch sculpture of seven strong spiral ridges well illustrated by Mordan (1998: 62, fig.
However, once the protoconch has formed, growth of the larval shell in pleurobranchids is atypical of other opisthobranchs in mechanism and timing.
This explains the fact that the egg dimensions cited above are somewhat larger than the protoconch dimensions given in the respective species treatments below.
The creases in the lateral flank of the protoconch correspond precisely with deep visceral clefts (indentations of the shell field epithelium) in the developing larva.
Protoconch diameter typically 7-8 mm, but its limit difficult to determine in many specimens.
At this time, the ovoid larval body has an encircling band of long, compound cilia (velar cilia) that powers swimming, but the foot is not yet recognizable and the protoconch (premetamorphic shell) is merely a small disc.