444 Daviclso7i and King — O^i the TrhnerelUdce. 



and massive, or compressed into a thin V-shaped plate ; in the former 

 condition 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 

 anything strictly resembling the partition in other Palliobranchs. In 

 Pentamerus, it is true, the umbonal cavity is divided by a medio-longi- 

 tudinal 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 muscular platforms, 

 of which an example occurs in each valve. A similar homologous 

 duplication characterizes other families — Pentamerids, Leptsenids, 

 etc. ; but this part generally occurs 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- 

 longitudinal septum. A tendency to the double-vaulting may be 

 observed in the great muscular supports of a few other Pallio- 

 branchs, particularly Leptcena Dutertrii, in which the ventral one 

 curves over and res.ts 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 presented 

 by the genus Oholus, in which certain muscular scars, usually 

 excavated, have an overlapping posterior margin : in Crania some- 

 thing similar is seen. The platform, with its tubular vaults and 

 biconvex surface, reminds one of a double-barrelled pistol. With 

 a pair of platforms of this kind associated, as is often the case, with 

 a couple of tubular umbonal chambers, the interior of a Trimerella 

 presents a singular appearance. In Monomerella both platforms are 

 solid and slightly raised ; and consequently the absence of vaults 

 gives the interior of this genus a totally different aspect : the umbonal 

 cavity, however, consists of two large chambers. Dinoholus has 

 neither a vaulted platform, nor a chambered umbo. Each of the 

 three genera contains species in which the platform varies con- 

 siderably, being reduced to so rudimentary a condition that it is 

 difficult to allocate them generically. Hall has been induced to 

 raise an aberrant species of the kind to the rank of a genus, 

 Bhjnobolus; but this step appears to us to be attended with consider- 

 able disadvantage, as it would necessitate instituting a genus for 

 every aberrant form. The scars are numerous and exceedingly com- 

 plicated by the modifications of the different parts, as just pointed 

 out. After some consideration we have abandoned the attempt to 

 hpmologize them, except in a few cases. We think the posterior 

 crescent, with its loop and lanceolate scars, corresponds to the post- 

 aponeural impressions in Lingula and Discina. We are unable to 



