436 



PTEROPODA — THECOSOMATA 



CHAP. 



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 : Limacina, shell helicoid, 

 deeply umbilicated (X. 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. — Illustrutious of Pteropoda Thecosomata: A, Limacina australis Eyd.; B, 

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

 Creseis vircfula Rang ; E, Cll') halantium Rang ; /,/, fins ; I, liver ; o, ovary ; 

 sh, shell. (After Souleyet.) 



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

 portion distinct, or formed of two separate plates. 



In Cavolinia Q=^ITyalaea, Fig. 5b, p. 10) the shell consists of 

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

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

 The aperture is broad, contracted dorso-ventrally. Two long 

 pointed prolongations of the mantle project from the lateral slits 

 of the shell, and probabl}^ serve to balance the bulky body when 

 swimming. Fins trilobed at the margin. Cleodora has only 

 rudimentary lateral prolongations, 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, Avhich is truncated 

 in the adult. The principal sub-genera of Clio are Cre-^eis, which 

 Jias an elongated sub-cylindrical shell, sometimes slightly curved, 



