ON THE PRINCIPLE OF CEPHALIZATION. 173 



Diameter. BRACHIOPODS. MONOMYARIANS. 



Antero-posterior. Medium. Small. 



Dorso-ventral. Small. Very large. 



Transverse. Large. Very small. 



For reason of their excessive narrowness, the greater 

 number of Monomyarians lie on the right or left valve, and 

 as their peculiar form precludes the possibility of locomo- 

 tion by the usual organ, the foot, they either remain fixed, 

 or swim freely about in the water, by violently closing 

 the'r valves, as in Lima and Pecten. 



Among the [Jnionidae, the highest family in the Lam- 

 . thp. animal assumes nearly a horizontal posi- 



EKKATA. 



Page 175, line 12 from bottom, for anterior pole read 

 posterior pole. 



Page 180, lines 1213 from top, for anterior end read 

 posterior end. 



know for a certainty which part hrst becomes axtacneu m 

 Vermetus and allied forms; their first point of attachment 

 must take place at the mouth of the tube or aperture, 

 which is really anterior and ventral). The Cephalopods 

 are free. 



Thus we have the various regions of attachment, chang- 

 ing and following in the direction indicated by the arrow 

 A, in Fjg. 1. 



1st, Polyzoa : dorsal attachment. 



2d, Brachiopoda : dorsal and anterior attachment. 



3d, Tunicata: anterior. 



4th, Lamellibranchiata : anterior and ventral attach- 

 ment. 



5th, Gasteropods : ventral and posterior attachment. 



