XV PTEROPODA — GYMNOSOMATA 437 



smooth or grooved ; and Clio proper, in which the shell is long, 

 angular, with a dorsal rib, apex ( = embryonic shell) rounded, 

 constricted. Styliola and HyalocjjUx also belong to this group. 



Fam. 3. Cymhuliidae. — Test (which is not homologous with 

 the shell of other Thecosomata) slipper-shaped, cartilaginoid, 

 simply a thickening of the mantle ; embryo with a calcareous, 

 spiral, operculate shell. Genera: Cymhulia, Cymhidiopsis^ G-leha. 



Three other families, Hyalithidae, Pterothecidae, and Conu- 

 lariidae, from Palaeozoic strata, are generally added to the The- 

 cosomata. All are fossil only, and it is doubtful whether they 

 are really Molluscan. Pelseneer holds that no true fossil Ptero- 

 poda occur until the lower Tertiaries. 



Section 2. Gymnosomata. — Mantle and shell absent in 

 the adult, fins not connected by a lobe, no branchial chamber, 

 head well developed, with two pairs of tentacles, eyes on the 

 posterior pair ; cerebral ganglia above the oesophagus ; buccal 

 cavity provided with a pair of protrusible "hook-sacs," radula 

 generally with 4 to 12 hooked laterals, central tooth triangular, 

 jaw in one piece, composed of horny plates, no horny plates in 

 stomach, anus on the right side. 



The Gymnosomata are carnivorous, feeding on Thecosomata 

 and other pelagic animals, being provided for this purpose with 

 a formidable buccal armature of hook-sacs and suckers. The 

 intestine, as usual in carnivorous groups, passes straight from 

 the stomach to the anus ; the fins are not attached to the head, 

 but to the anterior part of the body. The larva has a straight 

 shell, which disappears in the adult. About 21 species are 

 known, belonging to 7 genera. 



Fam. 1. Pneumodermatidae. — Animal fusiform, fins rather 

 small, head prominent, anterior part of buccal cavity protrusible, 

 with suckers on the ventral side, hook-sacs w^ell marked; 

 branchia on right side, skin soft, pigmented. Genera : Dexio- 

 hranchaea^ no posterior gill, hook-sacs short; Spongiobranchaea^ 

 posterior gill circular; Pneumoderma^ gill tetraradiate, hook- 

 sacs long. 



Fam. 2. Olionopsidae. — Body barrel-shaped, proboscis three 

 times the length of the body, no buccal appendages ; hook-sacs 

 short, no lateral gill, posterior gill tetraradiate, skin not pig- 

 mented. Clionopsis is the single genus. 



Fam. 3. Notohranchaeidae. — Body ovate, buccal appendages 



