250 Messrs. Davidson and King on the Genera 



what constant in form, it may, according to the species, be sub- 

 conical and massive, or compressed into a thin V-shaped 

 phite : in one genus it is obtusely rounded. In the first of 

 these conditions it may be solid or double-chambered : the 

 chambers are separated by either a thick or a thin partition ; 

 and they are shallow and wide-mouthed, or long and tubular. 

 We are not acquainted with any thing strictly resembling the 

 partition in other Palliobranchs. In Pentamevus^ it is true, the 

 umbonal cavity is divided by a medio-longitudinal plate, giving 

 rise to two lateral chambers : in this last genus, however, the 

 dividing plate is double, causing it, when a specimen is suitably 

 struck with the hammer, to split lengthwise into two halves ; 

 but no such division has occurred to us in any specimens of 

 Trimerellids. The undivided condition of the partition seems 

 to be explained on the view that this part is a modified form 

 of the hinge-wall. Passing to the parts seen in the general 

 or valvular cavity of the Trimerellids, the principal are the 

 great muscle-bearing platforms, of which an example occurs 

 in each valve. A similar homologous duplication characterizes 

 other families — Pentamerids, Lepteenids, &c. ; but the myo- 

 phores generally occur under a widely different shape. In the 

 typical genus of the present family the platforms are elevated 

 and doubly vaulted, the vaults being tubular and separated by 

 a partition. The latter part is continued beyond or in advance 

 of each platform, where it becomes the ordinary medio-longi- 

 tudinal septum. A tendency to double-vaulting may be ob- 

 served in the myophores of some other Palliobranchs, particu- 

 larly Leiitcena Dutertrii-^ in which the ventral one curves over 

 and rests upon the medio-longitudinal septum, forming thereby 

 a doubly vaulted arch. But the nearest approach to this 

 peculiarity, as pointed out by Billings, is undoubtedly pre- 

 sented by the genus Oholus, in which certain muscle-bearing 

 scars, usually excavated, have an overlapping posterior margin : 

 in Crania something" shnilar is seen. The platforms, with their 

 tubular vaults and biconvex surface, remind one of a double- 

 barrelled pistol. With a pair of this kind associated, as is often 

 the case, with a couple of tubular umbonal chambers, the inte- 

 rior of Trimerella presents a singular appearance. In Mono- 

 merella both platforms are solid and slightly raised ; and con- 

 sequently tlie absence of vaults gives the interior of this genus 

 a totally different aspect : the umbonal cavity, however, con- 

 tains two large chambers. Dmoholus has neither a vaulted 

 platform, nor a chambered umbo. Each of these three genera 

 contains species in which the myophores vary considerably, 

 being reduced to so rudimentary a condition that it is difficult 

 to allocate the species generically. Hall has been induced to 



