MICRODISCUS. 31 



the shales from which Emmons's specimen was obtained, belong to the Hudson 

 River group, which is of Ordovician age. The figure given by Emmons is 

 sufficiently clear to show that his specimen was one of the Trinucleidse, for the 

 tail, thorax, and glabella are all of the same type as in Trinucleus and Ampyx. 

 In the number of thoracic segments and the form of the head-shield, as shown in 

 the figure, the specimen undoubtedly presents immature characters, and, as 

 Trinucleus concentricus is abundant at the same horizon, it is probably the young 

 of that species. It cannot, therefore, stand as the type of a genus ; and if it did, 

 it is not the genus to which the species usually referred to Microdiscus belong. 

 How far the continuous usage of nearly forty years may override the strict rules 

 of nomenclature is a matter of personal opinion, but it would certainly be 

 premature at present to substitute another name ; for, as Walcott has pointed 

 out, Pemphigaspis bullata, Hall, 1 is closely related to the reputed Microdiscus, and 

 probably belongs to the same genus. Should this prove to be the case, Pemphi- 

 gaspis is the name which ought to be employed. Until, however, Hall's species is 

 more completely known, the matter remains uncertain, and for the present, 

 therefore, I follow other writers in using the name Microdiscus in Salter's sense, 

 Avith M. punctatus, Salter, as the type. 



The genus is usually distinguished from Agnostus by the facts that it possesses 

 three or four thoracic segments, and that the axis and sometimes the lateral lobes 

 of the tail are divided into a number of segments. It is less easy to give any 

 constant character which will invariably distinguish between the heads. In 

 Agnostus the glabella is usually divided into a small anterior and a long posterior 

 lobe ; in Microdiscus it is either smooth or more or less distinctly divided into 

 several segments of approximately equal length, and if the divisions tend to 

 become obsolete, it is in the anterior part of the glabella that they first disappear. 

 The basal lobes characteristic of Agnostus are absent in Microdiscus, or are 

 represented only by very obscure elevations in the axial furrows. In Microdiscus 

 the margin of the head is often beaded, punctate, or ornamented with tubercles ; 

 in Agnostus it is smooth. 



It is often stated that the thorax of Microdiscus consists of four segments. 

 Both M. punctatus, Salter, and M. speciosus, Ford,' 2 were originally described as 

 possessing four thoracic segments. Ford, 3 however, subsequently showed that in 

 M. speciosus the number of segments is really three ; and an examination of a 

 large series of specimens of M. punctatus proves that in that species also there are 

 only three. In M. punctatus each of the pleural segments is deeply grooved : 

 there is also a strongly marked groove near the posterior margin of the head, and 

 another near the anterior margin of the tail. There are thus five deep furrows 



1 Sixteenth Ann. Eep. N. Y. State Cab. Nat. Hist. (1863), p. 221, pi. v a, figs. 3—5. 



2 Amer. Journ. Sci., ser. 3, vol. vi (1873), p. 137. 



3 Ibid., vol. xiii (1877), p. 141. 



