Trimerella, Dinobolus, and Monomerella. 251 



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

 Ehynoholus ; but this step appears to us to be attended with 

 considerable disadvantage, as it Avould necessitate instituting 

 a genus for every aberrant form. The scars are numerous 

 and exceedingly complicated by the modifications of the dif- 

 ferent parts, as just pointed out. After some consideration we 

 have abandoned the attempt to homologize them, except in a 

 few cases. We think the posterior crescent, with its loop and 

 lanceolate scars, corresponds to the post-aponeural impressions 

 in Lingida and Discina. We are unable to specify which 

 scars have been produced by the valvular muscles, except 

 some situated on the platforms : and with respect to 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 em- 

 ploying terms 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 of the ventral one by a different type. Certain scars, or 

 other parts, apparently occupying the same relative positions 

 in the two valves, and which appear to be analogous, bear 

 the same letter, but in a different type : nothing more is meant 

 by this mode of lettering. 



The geographical distribution of the Trimerellids is a matter 

 of some importance. Eminently a Silurian gToup, one might 

 have expected the well-explored region ^vhich the labours of 

 Murchison have made classical would have yielded an abund- 

 ance of examples ; but it is remarkable that only a few speci- 

 mens of a single genus, Dinoholus^ and a]:)parently 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 gi'cater variety of the family occurs rather abundantly 

 in rocks of the " Aymestry " age of that remarkable locality. 

 Canada and adjacent districts in the United States have yielded 

 the greatest variety of species, all of which, with the exception 

 of Dinoholus canadensis and D. magnijica, are referable to 

 the Upper Silurians. The two species last named occur in 

 the Black-River limestone, a rock which appears to be equi- 

 valent to the Upper Llandeilo, or to the base of the Caradoc 

 of this country. A species of Monomerella has also been found 

 in Livonia (Russia) in rocks corresponding in age with those 

 in which the same genus occurs in Gothland. 



Our labours on the Trimerellids have enabled us to confirm, 

 for the most part, the conclusions of j^revious Avriters as to 



19* 



