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 Hyalocylix also belong to this group. 



Fam. 3. Ci/mhuliidae. — 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 : Cymliulia, Cymhuliopsis, Gleba. 



Three other families, Hyalithidae, Pterothecidae, and Conu- 

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

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

 really Molluscan. Pelseneer holds that no true fossil Pteropoda 

 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 pro- 

 vided 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 Tliecosomata 

 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 well marked ; 

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

 hrancJiaea, no posterior gill, hook-sacs short ; Spongiobranchaea, 

 posterior gill circular ; Pneumoderma, gill tetraradiate, hook-sacs 

 long. 



Fam. 2. Clionopsidae. — 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. Cliono'psis is the single genus. 



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



