396 Dr. 0. A. L. Morch on the Classification of the Mollusca. 



a single auricle, although the gills are lateral — arid because Turbo mar- 

 moreus, with a single gill, has two auricles, according to Quoy and 

 Gaimard.) Sexes distinct, but without male organ. 



Class III. PSEUDOPHALLIA (Aspidobranchia olim). 



Tongue with the lateral teeth of two different forms. Male organ 

 rudimentary ? Development only known in Chiton and Dentalium. 



Rhii'IDOGLOSSATA. Median teeth broad, with reflected edge, ge- 

 nerally 5.1 .5 ; marginal teeth compressed ; these are very numerous, 

 with inflected tips. 



Terrestria. Helicina. (Eyes sessile.) 

 Fluviatilia. Neritina. (Eyes petiolate.) 



Marina. Nerita, Turbo, Trochus, Huliotis, Fissurella, Emar- 

 ginula. (Eyes sessile.) 



Heteroglossata. (Dochoglossata, Troschel). Tips of the teeth 



(always?) having a black pigment. The marginal teeth of the 



preceding division are absent. 



Cyclobranchia. Patella, Tectum. 



Polyplacophora. Chiton, Chitonellus. 



Cirrobranchia. Dentalium, Siphonodentalium. 

 Cephalopoda. 



1. Dibranchiata. 

 A. Octopoda. 

 13. Decapoda. 



Oigopsidae. 

 Myopsidae. 



2. Tetrabranchiata. Nautilus. 



Class IV. ACEPHALA (Dithyra). 

 Dimyaria. 



Heteromyaria. Mytilacea et Ostreacea! 

 Monomyaria. Pecten, Spondylus, Lima, Tridacna. 



Observations. 



Ianthina has perhaps the same relation to Pneumodei-mon as 

 Carinaria to Pterotrachcea. Ianthina has a pair of epipodial fins. 

 Dr. Gould has represented (American Exploring Expedition) 

 some aeiculate bodies, probably homologous with the hooks on 

 the arms of Pneumodermon. The latter genus lias, according to 

 D'Orbigny, a small hump on the back, probably a rudimentary 

 mantle ; but intermediate genera are yet unknown. 



The Thecosomata differ considerably from the Gymnosomata. 

 I find it at least very doubtful whether Eurybia makes a true 

 transition between the two divisions. The'Thecosomata have 

 3-5 band-like lateral mandibles, and atriscriate lingual dentition, 

 quite different from that of the Gymnosomata. According to 

 Soulcyet, they have a petiolate bladder, and are consequently 



