TYPE MOLLUSC A. 365 



IV. Class Pelecypoda. — Visceral hump not developed; bilaterally sym- 

 metrical; mantle forms two lateral folds; shell bivalved; anus 

 terminal. 



1. Order Pi-otobranchia. — Gill a true ctenidium; pleural ganglia not 



united to cerebral. Ifucula, Yoldia. 



2. Order Filibranchid. — Gill-filaments elongated and bent upwards 



at ends; cerebral and pleural ganglia fused. Anomia, Mytilus, 

 Modiolaria, Area. 



3. Order Pseudolamellibranchia. — Gill-filaments turned up at ends 



and with interlamellar junctions; cerebral and pleural ganglia 

 united. Pecten, Ostrea. 



4. Order Eulamellihranchia.—G\\\ filaments united to form a plate- 



like gill; cerebral and pleural ganglia united. Venus, My a, 

 Ensatella, Pholas, Teredo, Unio, Anodon, Cyclas. 



5. Order Septibranchia. — Gill reduced to a muscular perforated sep- 



tum between the mantle and suprabranchial chambers. Cus- 

 pidaria. 

 V. Class Cephalopoda. — Visceral hump developed; bilaterally symmetri- 

 cal; mantle a circular fold; foot (propodium and mesopodium) 

 forming arm-like structures provided with suckers and sur- 

 rounding the mouth. 



1. Order Tetrabranchia. — With four etenidia and with external 



chambered shell. Nautilus. . 



2. Order Dibranchia.— With two etenidia ; shell if external not 



chambered, usually internal. 

 With eight arms to foot {Octopoda). Argonauta, Octopus, Trem- 



octopus, PMlonexis. 

 With ten arms to foot {Decapoda). Spirula, Sepia, Loligo, 



Ommastrephes. 



LITERATURE. 



GENERAL. 



Gould and W. G. Binney. Tlie Invertebrata of North America. Boston, 1870. 



G. W. Tryon and H. A. Pilsbry. Manual of Conchology. In course of publi- 

 cation. Philadelphia. 



S P. Woodward. A Manual of the Mollusca. 3d ed. London, 1878. 



E. R. Lankester. Article "Mollusca." Encyclopaedia Britannica, 9tb Ed. 

 London, 1883. 



W. Patten. The Eyes of Mollusks and Arthropods. Mitth. a. d. Zool. Station 

 Neapel, VI, 1886. 



AMPHINETIRA. 



B. Haller. Bie Organisation der Ohitonen der Adria. Arbeiten a. d. Zoolog. 



Institut Wien, IV, 1883 ; v, 1883. 

 H. N. Moseley. On the Presence of Eyes in the Shells of certain Chitonidm and 



