Trimerella, Dinobolus, and Monomerella. 249 



sliglitlj calcareous nature, as was generally the case with 

 their contemporaneous 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 essentially a calcareous frame- 

 work, as was the case with a vast number of their coeval orga- 

 nisms, seems to show not only that such compounds had 

 increased in the Silurian seas, but further to support the con- 

 clusion that the family we are engaged with is a post-genetheo- 

 nomic branch of the Lingulids. With the physical changes 

 indicated, the shells of the present family underwent important 

 modifications compared with the group from which they pre- 

 sumedly 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 correction, the ventral or rostral valve characterized 

 by possessing twenty-four different parts, their dorsal one 

 having sixteen. Many of the parts are so unlike what are seen 

 in other families as to defy all attempts to determine their uses 

 or functions. One consideration that strikes us forcibly is that 

 such 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 car- 

 dinal process may be a thick projecting lamina, or rude in 

 shape and massive, or absent altogether. The deltidium 

 seems to be less liable to modifications : 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. The usual areal border lies on the outside 

 of each of the deltidial ridges. The deltidium itself is, in 

 general, wide and transversely marked with strong lamina-like 

 lines : 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 forth- 

 coming memoir it will be shown that another part, the deltidial 

 slope, further testifies to the close affinity between the Trime- 

 rellids 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 indi- 

 viduals. The hinge-wall, as will shortly be seen, is equally 

 subject to variation. The umbo or beak, which is usually 

 prominent, presents itself under different appearances. Some- 



Ann.& Mag. N. Hist. Ser.4. TW. x. 19 



