OF MOLLtTSCA. 191 



Subclass II. HETEROBRANCHIATA, Gray. 



Respiratory organs variously formed. Gill exposed, or only 

 slightly covered by a fold of the mantle, or in the form of a lung- 

 like cavity. Hermaphrodite, with reciprocal impregnation (p. 3). 



Heterobranchiata, Gi'ay, 1840. 

 Opisthobranchiata et Pulmonata, M.-Edw. 1846. 



Order III. Pleurobranchiata. 



Gill forming a tuft on the side of the back under a fold of the 

 mantle. Shell spiral. Adult and larva shell-bearing ; larvae fur- 

 nished with deciduous cephalic fins. Animal hermapbrodite, with 

 reciprocal impregnation. Animal and foot elongate, for walking. 



A. Organs of generation far apart, united by a groove. Tbctibranchia, 



Rang, 

 a. The tentacles broad, forming a more or less expanded frontal 

 disk. Bullinina. 



" The head-lobe in the Bullidee is in fact nothing else than the dor- 

 sal and labial tentacles fused into one continuous mass. This Cuvier 

 asserted long ago, and it can very easily be proved on anatomical 

 grounds. The nerves which supply the oral and dorsal tentacles in 

 the Gasteropods go to this lobe, the former to the anterior, the latter 

 to the posterior portion of it (the olfactory nerves to the upper part 

 of it, hence it is the organ of scent)." — Hancock, Ann. fy Mag. Nat. 

 Hist. 1852, 188. 



" In Bulla hydatis the olfactory organ is most highly developed, 

 and is perfectly similar in structure to that observed in fishes, and 

 shows the analogy that exists between the dorsal tentacle of Boris 

 and the olfactory organs of this higher type of form. It is composed 

 of a central stem, bearing numerous considerably elevated lateral 

 laminae. Mr. W. Clark, who described it (Ann. fy Mag. Nat. Hist. 

 1850, vi. 100), erroneously considers these 'leaflets*' salivary glands." 

 —Hancock, Ann. fy Mag. Nat. Hist. 1852, 190. 



§ Teeth in two or four longitudinal series. 



Fam. I. PHILINIDiE. 



Teeth in two (or rarely four) longitudinal series, large, hooked, 

 sometimes with a series of smaller similar teeth on the outer side. 

 Head-disk square, broad. 



* Shell none. 



1. ? Gasteropteron. 

 Head-disk triangular ; eyes sessile, subcentral ; body ovate, shell- 



