TkENTON LIMESTONE. 131 



Genus DELTHYRIS, or SPIRIFER.* 



We have but a single species of this genus in our older strata, so far as known at the 

 present time ; and this one possesses so many abnormal characters, that M. de Verneuil 

 is inclined to regard it as distinct from the true Spirifer, in having, always, the ventral 

 larger than the dorsal valve. 



There are also some other reasons why this species may be regarded as an erratic one, 

 possessing in part the characters of both Orthis and Delthyris. Wc shall have observed, 

 in some of the preceding species of Orthis, a remarkable departure from the general 

 external forms of this genus, and an approach to the Delthyris ; this variation in external 

 form is also accompanied by a difference in the internal characters. The area and foramen 

 being common to both valves, is a character of frequent occurrence in Orthis, but rare in 

 the Delthyris, where the ventral valve of normal species scarcely presents the vestige of 

 an area or foramen. 



It is very remarkable that the single species known in our older strata, and apparently 

 the earliest created one both on this continent and in Europe, should present characters 

 common to the Orthis, which is so well characterized, and the species of which are very 

 numerous in our older rocks. The Delthyris, we shall observe, also, is not one of the 

 earliest developed forms ; for in the first epoch when Brachiopods occur, we find Lept^na, 

 Orthis and Atrypa ; while in the epoch of which we are now speaking, we find these 

 three forms largely developed, with a single one assuming the delthyroid type, which, in 

 subsequent periods, becomes so numerous in species. 



Glancing for a moment at the Brachiopods in their successive appearance upon our globe, 

 we find that Lept^na, Orthis and Atrypa, appear at the same period ; and these must be 

 regarded as types in the order of time, and as those to which we should rCally look for 

 our ideas of generic character. In the second period, or the one of which we are now 

 speaking, we find the orthoid type has admitted of some modifications, and that the first 

 delthyroid type possesses some of those characters which are thrown off, and the true normal 

 character assumed in subsequent periods. ^ 



If we choose to look a little farther, we shall find the leptsenoid forms furnished with 

 spines on the cardinal margin ( the Chonetes, Fischer) ; and, finally, that as the surface 



* I have preferred to use the term Delthyris instead of Spirifer, though both names have at the present time the 

 same application. The genus Spirifer was proposed by Mr. Sowehby, ten years in advance of Dayman's sub- 

 diviaions of the Bbacuiopoda ; but the characters given by the former do not separate that genus from Orthis, and 

 not only have specimens of that genus, but also of atrypa or Terebratula, been confounded under the same term. 

 The spiral appendage within the shell, which gives origin to the name, is found likewise in several species of Atrypa 

 in the palaeozoic strata ; and this therefore cannot be regarded as a distinguishing character, any more than the deltoid 

 foramen in Delthyris, which is equally common to Orthis Since, however, the application of the two names is now 

 precisely similar, either one may be used without producing confusion ; and I am well satisfied that a subdivision of 

 the genus Spirifer (as now limited in Europe) will be made, when the species, varying so much in external character, 

 shall bs more carefully studied. 



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