MOLLTTSCA. 155 



Of the three living genera only a very few species are known ; 

 one Crania, with two species is found in European seas. Lin- 

 gula, anatina and two or three other species, now confined to the 

 Eastern seas, do not differ morphologically from their congeners 

 of the Cambrian epoch. Discina, of which there are several spe- 

 cies, is found in China, the West Indies, and the Pacific coasts ; 

 it was also existent in the Silurian epoch. 



Lingulidce. Discinida. Craniidcs. 



Lingula. Discina. Crania. 



*Obolus. *Trematis. 



Order II. ARTHROPOMATA. 

 TESTICARDINES. APYGIA. ARTICULATA. SCLEROBRANCHIATA. 



Valves held together by teeth. Shell calcareous. Intestine 

 without an anus. 



A peculiar calcareous loop is attached to the upper valve of 

 most of the species, destined for the support of the arms. The 

 alimentary canal is short, with a cascal end. 



The recent species, mostly confined to deep water, are found in 

 almost all parts of the world. The Terebratulidae are commonly 

 known as "lamp-shells." 



Terebratulidce. Spiriferidee. Orthidce. 



Terebratula. Spirifer. *Orthis. 



Waldheimia. *Koninckia. 



Kraussia. *Spirigera. Productidce. 



*Productus. 



i. BhynchonellidcB. *Chonetes. 



Rhynchonella. 



Class II. LAMELLIBRANCHIATA. 

 BIVALVIA. CONCHIFERA. ACEPHALA. DITHYRA. CORMOPODA. 



Headless mollusks encased in a bivalve shell, sometimes with 

 accessory valves. Body enclosed within a mantle. Respiratory 

 organs consisting of lamelliform or filamentous branchiae. Sexes 

 distinct. 



The branchiae are usually two on each side, placed between the 

 mantle and the foot, and well supplied with cilia. The mouth 



