Davidson and King — On the Trmerellidce. 



445 



specify wliich scars have been produced by the valvular muscles, 

 except some situated on the platforms ; and in respect of the latter, 

 our efforts to identify them with the valvulars of Lingula (the 

 nearest living representative as we believe) have not, it is to be 

 apprehended, been attended with much success. We have for the 

 reasons stated refrained as far as possible from employing names 

 for the different scars implying their uses ; and have, instead, simply 

 given them names denoting their relative position, distinguishing 

 the group in the dorsal valve from that in the ventral one by letters 

 of a different type. Certain scars, or other parts, apparently occupy- 

 ing a similar relative position in the two valves, and which appear 

 to be analogous, bear the same letters, but in a different type. 



The geographical distribution of the Trimerellids is a matter of 

 some importance. Eminently a Silurian group, one might have ex- 

 pected that the well-explored region, which the labours of Murchison 

 have made classical, would have yielded an abundance of examples ; 

 but, it is remarkable, only a few specimens of a single genus, 

 Dinobolus, and apparently the last of their race, have been met with 

 in the Wenlock limestones and shales near Dudley, and discovered 

 for the first time in 1852. Identical deposits in Gothland contain 

 the same species : but a greater variety of the family occurs 

 rather abundantly in rocks of the "Aymestry" age of that re- 

 markable locality. Canada and adjacent districts in the United 

 States have yielded the greatest variety of species, all of which, 

 with the exception of Dinobolus Canadensis and D. magnifica, being 

 referable to the Upper Silurians. The two species last named occur 

 in the Black river limestone, a rock which appears to be equivalent 

 to the Upper Llandeilos, or to the base of the Caradocs of this 

 country. A species of Monomorella has also been found in Livonia 

 (Russia) in rocks corresponding in age with those in which the 

 same species occurs in Grothland. 



Our labours on the Trimerellids have enabled us to confirm, for 

 the most part, the conclusions of previous writers as to the number 

 of species, and to determine the existence of some others. The 

 three genera are severally constituted in species as follows : — 



Trimerella grandis, Billings. 



acuminata, Billings. 

 Lindstromi, Dall. 

 BiUingsii, Dall. 

 Ohioensis, Meek. 

 Dalli, Dav. and King. 

 Wisbyensis, Dav. and King. 

 Dinobolus Conradi, Hall. 



„ Canadensis, Billings. 



Dinobolus Galtensis, Billings. 

 „ Davidsoni, Salter. 



„ transversus, Salter. 



„ TFoodwardi, Salter. 



„ magnifica, Billings. 



Monomerella Walmstedti, Dav. & King. 

 „ prisca, Billings. 

 ,, orbicularis, Billings. 



With one or two exceptions, all the species will be fully illustrated 

 in five lithographic plates in our forthcoming memoir : in addition 

 to which there will be two wood-cut plates of diagram-figures, ex- 

 plaining the various parts briefly noticed on the present occasion, and 

 another showing the relationship of Lingula to the family. 



