TYPE MOLLUSC A. 365 



IV. Class PELECYPODA. Visceral bump not developed; bilaterally sym- 

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

 terminal. 



1. Order Protobranchia. Gill a true ctenidium; pleural ganglia not 



united to cerebral. Nucula, Yoldia. 



2. Order Filibranchia. 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. Pecteu, Ostrea. 



4. Order Eulamellibranchia.GriM filaments united to form a plate- 



like gill; cerebral and pleural ganglia united. Venus, Hya, 

 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 mesopodiutri) 

 forming arm-like structures provided with suckers and sur- 

 rounding the mouth. 



1. Order Tetrabranchia. With four ctenidia and with external 



chambered shell. Nautilus. 



2. Order Dibranchia. With two ctenidia ; shell if external not 



chambered, usually internal. 

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



octopus, Plrilonexis. 

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



Ommastrephes. 



LITERATURE. 



GENERAL. 



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



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

 cation. Philadelphia. 



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



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

 London, 1883. 



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

 Neapel, vi, 1886. 



AMPHINETJRA. 



B. Haller. Die Organisation der Chitonen der Adria. Arbeiten a. d. Zoolog. 



Institut Wien, iv, 1882 ; v, 1883. 

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



