436 



PTEROPODA THECOSOMATA 



they have no proboscis, and the intestine is flexured. The fins 

 are always closely connected with the head, or what answers to 

 it. About 42 species are known, belonging to 8 genera. 



Fam. 1. Limacinidae. — ^Fins very large, branchial chamber 

 dorsal, anus on right side ; shell spiral, sinistral (ultra-dextral, 

 see p. 249), operculate. Genera : Liviacina, shell helicoid, 

 deeply umbilicated (Z. helicina swarms in Arctic seas and 

 furnishes food for many Cetacea) ; Peraclis, spire turreted, 

 aperture large, elongated, produced anteriorly, no umbilicus ; 

 operculum sinistral, in spite of the shell being ultra-dextral. 



Fam. 2. Cavoliniidae. — Fins large, branchial chamber ventral. 



Fig. 290. — Illustrations of Pteropocla Thecosomata : A, Limacina australis Eyd.; B, 

 Cleodora cuspidata Bosc. (shell only) ; C, Cuvierina columnella Piang ; D, Crescis 

 virgida Rang ; E, Clio balantium Rang ; /, /, fins ; I, liver ; o, ovary ; sh, sliell. 

 (After Souleyet.) 



shell a non-spiral cone, angular or round, very thin, emljryonic 

 portion distinct, or formed of two separate plates. 



In Cavolinia ( = Hyalaea, Fig. 5, B, p. 10) the shell consists of 

 two plates, the ventral being convex, with one to three sharp 

 spines at the posterior end, the dorsal flatter, without spines. 

 The apertm-e is broad, contracted dorso-ventrally. Two long 

 pointed prolongations of the mantle project from the lateral slits 

 of the shell, and probably serve to balance the bulky body when 

 swimming. Fins trilobed at the margin. Cleodora has only 

 rudimentary lateral pi-olongations, fins bilobed, shell triangular, 

 angles greatly produced, aperture very wide, dorsal side keeled. 

 In Cuvierina the shell is straight, sub-cylindrical, with a median 

 partition, slightly expanding towards the apex, which is truncated 

 in the adult. The principal sub-genera of Clio are Creseis, which 

 has an elongated sub-cylindrical shell, sometimes slightly curved. 



