Davidson and King — On the Trimerellidce. 443 



and Ehynchonellids that succeeded them, the conclusion suggests 

 itself that the latter and simpler groups are the degraded successors 

 of a type that existed in the earliest known Life-period of our 

 planet. 



Another matter for consideration is the fact that the Cambrian 

 Lingulids were furnished with a framework of a horny or slightly 

 calcareous nature, as was generally the case with their contempo- 

 raneous Coelenterates and Crustaceans, making it doubtful that 

 ordinary marine calcium compounds were important solutions in the 

 seas of their period; while the fact that the Trimerellids had essenti- 

 ally a calcareous framework, as was the case with a vast number of 

 their coeval organisms, seems to show not only that such compounds 

 had increased in the Silurian seas, but further to support the conclu- 

 sion that the family we are engaged with is a post-genetheonomic 

 branch of the Lingulids. With the physical changes indicated, the 

 shells of the Palliobranchs under notice underwent important 

 modifications compared with the group from which they presumedly 

 originated. 



The Trimerellids are strongly differentiated by the variety and 

 form of their parts. The species, in general remarkably distinguished 

 by their massive umbonal region, have, speaking subject to correc- 

 tion, the ventral or rostral valve characterized with twenty-four 

 different parts, and their dorsal one with sixteen. Many of the 

 parts are so unlike what are seen in other families as to defy all 

 attempts to determine their use or function. One consideration that 

 strikes us forcibly is that parts, as the teeth, and cardinal process, — 

 essentials in other Palliobranchs, — are exceedingly mutable, not only 

 in a genus, but in a species : besides, they are rarely well defined. 

 The teeth may be large and crude in certain individuals, but rudi- 

 mentary or obsolete in others of the same species. The cardinal 

 process may be a thick projecting lamina, or rude in shape and 

 massive, or absent altogether. The deltidium seems to be less liable 

 to modification : situated on a well-developed area, it is bounded by 

 two rather prominent ridges, one on each side, with their inner and 

 projecting terminations serving as teeth ; and having the usual 

 areal border on the outside of each of the deltidial ridges. The 

 deltidium itself is, in general, wide, and transversely marked with 

 strong lamina-like lines of growth : it presents the appearance of 

 being excavated out of the areal face (or underlying solid portion) 

 of the beak, agreeing in this respect with what obtains in Lingula. 

 In our forthcoming memoir it will be shown that another part, 

 the deltidial slope, further testifies to the close affinity between the 

 Trimerellids and the last-named genus. The hinge or cardinal plate, 

 which requires more explanation than can be given on the present 

 occasion, is so variable in one species (Trimerella Lindstromi) as to 

 be with difficulty recognized in some individuals. The hinge-wall, 

 as will shortly be seen, is equally subject to variation. The umbo 

 or beak presents itself under different appearances ; being in one 

 genus obtusely rounded and in the others remarkably prominent. 

 Somewhat constant in form, according to species, it may be subconical 



