TRENTON LIMESTONE. 131i 
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. pp VeRNevIL 
is inclined to regard it as distinct from the true Sprrirer, 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 Orruts and Detruyris. We shall have observed, 
in some of the preceding species of Orruts, a remarkable departure from the general 
external forms of this genus, and an approach to the DeLruyris; 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 Orruis, but rare in 
the De.ttuyris, 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 Ortuts, which is so well characterized, and the species of which are very 
numerous in our older rocks. The Detruyris, we shall observe, also, is not one of the 
earliest developed forms ; for in the first epoch when Brachiopods occur, we find Lepraya, 
Ortuis and Arrypa; 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 Leprana, Ortuisand Arrypa, appear at the same period ; and these must be 
regarded as types in the order of time, and as those to which we should really 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 lepteenoid forms furnished with 
spines on the cardinal margin ( the Chonetes, Fiscuer); and, finally, that as the surface 
*]T have preferred to use the term Delthyris instead of Spirifer, though both names have at the present time the 
game application. The genus Spirifer was proposed by Mr. SowErsy, ten years in advance of DALMAN’s sub- 
divisions of the BRacuiopopa; 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 paleozoic 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 be more carefully studied. 
Lie 
