CLASSIFICATION. 259 



Class 1. EXOCEPHALA, Latr. (Pseudophallia, Morch). 



Order 1. Rhipidoglossa. Troschel. Proserpina, Helicina, 

 Hydrocena, with sessile eyes. 



Gray, in figuring the teeth of Proserpina (Ceres), formed for 

 this mollusk a distinct suborder, alongside of the Neritinse, which 

 he called Pseudobranchia. In the same year (1857) Morch 

 placed Helicina? in the Rhipidoglossates, with Neritina, notwith- 

 standing the want of an operculum in Proserpina, an example 

 followed by Troschel with some hesitation. 



Eyes sessile. Shell not nacreous. Neritina, Nerita. 



Shell nacreous. Phasianella, Turbo, Trochus, Margarita, 

 Stomatia, Haliotis. 



Eyes not petiolate. Shell not nacreous. ? Scissnrella, Emar- 

 ginula, Fissurella, the affinity of which with Haliotis, is incon- 

 testable. 



Order 2. Heteroglossata, Gray. (Orthodonta, Morch, Doco- 

 glossa, Troschel. 1 Patella, Tectura, Pilidium, etc., Chiton. 



Class 2. ACEPHALA, Cuvier. (Endocephala, Latreille;' 

 Dithyra, Anst.) 



Dimyaria (Plagymiona, Latr.). 



Heteromyaria (Mytilacea). 



Monomyaria (Mesomyona, Latr.). 



Notwithstanding that the systematic value given above to 

 these different groups varies from that of other authors, their 

 order of succession differs but little from Cuvier. The separa- 

 tion of the Rhipidoglossata (Pectinibranchiates) and Scutihran- 

 chiates, is the principal change. The arrangements of Gra}"^ and 

 Troschel differ still more, above all in the Androgjaia, which are 

 placed betw^een the Heteroglossa and Acephala, probably because 

 androgynism is considered as a character of inferiority ; notwith- 

 standing that the Acephala, which are inferior, have the sexes 

 separate, with some exceptions. The little division, Ptenoglossa, 

 including only Janthina, Scalaria, Solarium, is placed in the 

 system of Troschel, between the Rhachiglossa and Rhipidoglossa. 

 If we admit a special concordance between the teeth of these 

 three groups, Morch still does not consider the difference suffi- 

 cient to justify a separation from the Androgyna. Janthina 

 appears to him more close to the Pteropods by its lateral wings, 

 and Scalaria, notwithstanding the position of the e^^es, approaches 

 Chemnitzia. 



As in the entire animal kingdom, the greatest difference exists 

 amongst authors relative to the value of the swimming organs. 

 Latreille has united the Cephalopods and Pteropods in a single 

 division Pterygia, to which he attributes the same value as to 

 his Apterygia, comprehending all other mollusks. Gegenbauer 



