136 



which will be specified in the account of each class : if degree of organization bo 

 considered to depend upon the restriction of organs to particular functions of 

 life, this classiiicatiou will be in descending order. 

 Class I. Cephalopoda. 

 II. Gasteropoda. 



III. Ptebopoda. 



IV. Lamellibeanchiata, 

 v. Brachiopoda. 



VI. TUNICATA. 



VII. Bryozoa. 



The Tunicata (Heterobranchiata, Blainville), having no hard appendage, 

 are beyond the reach of geological investigation. 



There is some doubt about the right of the Bryozoa (Polyzoa, Thompson) 

 to a place in this sub-kingdom. Some natiu-alists rank them with the Radiates ; 

 others, again, regard them as forming with the Tunicata a transitional group, to 

 ■which they have given the name Molluscoids. 



For the convenience of those who may consult " Siluria" with reference to 

 this paper, it may be mentioned that the subdivision of the Mollusca adopted 

 there is as follows : — 



Province— ODONTOPHOKA. 

 Class— Cephalopoda. 

 ,, Ptebopoda. 

 ,, Heteropoda. 

 ,, Gasteropoda. 



Province— LAMELLIBKANCHIATA. 



Class — CONCHIFERA. 



Province— MOLLUSCOID A. 

 Class — Brachiopoda. 

 ( ,, Tunicata. ) 

 ,, Polyzoa. 

 This classification, which agrees in the main with that of Prof. Huxley 

 (Comparative Anatomy, London, 1864), is one of the best hitherto suggested. 



The best manual for the study of the Mollusca is Woodward's Becent 

 and Fossil Shells (London, Weale) ; we shall follow his arrangement in the 

 subdivision of the several classes, extinct families being indicated in the lists 

 by Italic type,those which have representatives in Silurian rocks by an asterisk. 



CLASS CEPHALOPODA. 

 Order i. — Dibranchiata. 

 Beleninitidcii 



Ai'gona.utidoe 



Octopodidse 



Teuthidfe 



Sejuadse 

 Spirulidae 

 Order ii. — Tetrabkaxchiata. 



"Nautilidse I Ammonitidce 



"Orthoceratidse 



