78 CLASSIFICATION OF THE ANIMAL KINGDOM. 



There are great difficulties in the way of arriving at any final conclu- 

 sion as to the systematic position of the three groups of the Polyzoa, Tuni- 

 cata, and Brachiopoda. Many high authorities now regard the Polyzoa as 

 an aberrant group of worms, related to the true Annelides, the grounds 

 for this conclusion being mainly derived from a study of the develop- 

 mental history of the Polyzoa. Upon similar grounds, many naturalists 

 consider the Brachiopoda as also a modified group of the worms. Lastly, 

 the Tunicata are often looked upon as a degraded type of the Vertebmta. 



DIVISION B. MOLLUSCA PEOPEE. 

 CLASS I. LAMELLIBRANCHIATA (CONCHIFERA). 



SUB-CLASS I. ASIPHONIDA. 



ORDEK I. OSTREACE.E. 



Fam. a. Anomiadse. -Anomia. 

 Fam. b. Ostreidae. Ostrea, *Gryphcm. 

 Fam. c. Placunidse. Placuna. 

 Fam. d. Pectinidse. Pecten (Scallop). 

 Fam. e. Limadse. Lima. 



Fam. /. Spondylidse. Spondylus. 



ORDER II. MYTILACE.E. 



Fam. a. Aviculidse. Avicula (Pearl-oyster), Malleus, 



*Inoceramus. 



Fam. b. Mytilidse. Mytilus (Mussel), Dreissena. 

 Fam. c. Pinnidse. Pinna. 



ORDER III. ARCACE^E. 



Fam. a. Arcadse. Area, Pectunculus. 



Fam. b. Nuculidse. Nucula. 



Fam. c. Nuculanidse (Ledidae). Nuculana (Leda)< 



Fam. d. Trigoniadee. Trigonia. 



ORDER IV. UNIONACE^E. 



Fam. Unionidas. Unio, Anodon (Fresh-water Mussels), 



