146 Zoological Society. 



as the cerebellum in the Sharks, Sawfish, &c. (Owen's Lectures, ii. 

 1 75) ; or the very diminutive size of a segment, as the cerebellum in 

 many reptiles ; or the coalescence of the pair, and consequent obli- 

 teration of the mesial division, just as is equally the case between the 

 two halves of the spinal cord, as in the cerebellum. 



3. Embryonic anatomy, too, comes in to strengthen the conclusion 

 of comparative anatomy, that a series of four ganglia is the essential 

 element of the brain, and that all the other parts of which it consists 

 in adult life of the higher Vertebrata, including of course the cere- 

 brum, are superadded. 



The argument of the preceding sections, exclusive of Section I., and 

 the conclusion to which it is intended to lead, may thus be stated : — 



Considering that the head of the Insecta, Myriapoda, &c. is com- 

 posed of a series of segments serially homologous with those of the 

 body, as its brain is of ganglia serially homologous with those of the 

 cord ; that a vertebra is the general homologue of a segment as the 

 spinal cord is of the ganglionic cord ; and that the brain of the Ver- 

 tebrata consists of a series of four segments ; there appears a strong 

 probability that its head in like manner shall consist of a series of 

 four vertebrae. 



3. Monograph of the species of Myochama, including 



THE descriptions OF TWO NEW SPECIES FROM THE COL- 

 LECTION OF H. Cuming, Esq. By Arthur Adams, R.N., 



F.L.S. ETC. 



Myochama, Stutchbury. 



Testa ineequivahis, adhcerens ; vaha affixa dentibus diiobus mar- 

 ginalibus, divaricatis, ad umbonem disjunctis, foveold trigond 

 intermedid alteram testacece appendicis extremitatem, cartila- 

 yine carried connexam, excipiente ; valva libera dentibus duobus 

 incequalibus, parvis, divaricatis, alterd appendicis extremitate 

 foveolce intermediee insertd ; umbones valves liberce internh, alte- 

 rius externe, recurvi ; impressiones muscidares duee orhicidares, 

 distantes, laterales; impressio mttscularis pallii sinu brevi lata, 

 ligamentum tenue externum. 



Myochama anomioides, Stutchbury. M. testa rosed, tenia, 

 fragili, costis prominentibus radiantibus dicTiotomis ; valvd li- 

 berd valde convexd ; umbone extra apicem valcce alterius pro- 

 diicto ; epidermide tenui pellucidd. 

 Long. W ; lat. ^ ; alt. ^. 

 Hab. 



This species is always regularly radiately ribbed, but when found 

 attached to smooth shells the ribs are smooth, but if fixed to Trigo- 

 nia pectinata they are crossed by tubercles. 



Myochama transversa, A. Adams. M. testd incequilaterali 

 transversa fused, subquadratd, antich longiore postice breviore 

 subtruncatd, radiatim costatd, costis subnodosis interdum di- 



