20 PALEONTOLOGY OF NEW YORK. 



strong senile development of a disappearing type. It has already been re- 

 marked that the term Delthyris, Dalman, may properly be restricted to the 

 unicispinate Fimbriati. The name was applied to species of which S. elevatus, 

 Dalman, was the first and typical member, a form not unlike S. crispus, Hisinger, 

 and from faunas of the same age. 



2. Duplicispinei = RETKVLAniA, McCoy, 1844; those species in which the 

 fimbriae are composed of large, compound, hollow spines, often or always with 

 lateral branches. Each spine is divided medially by a vertical septum, and 

 along this line the spine is depressed exteriorly, giving it a double-barreled 

 appearance ; from each lateral margin are given off at regular intervals short 

 spinules at right angles to the main spine. 



This peculiar structure was first described by Mr. Davidson, in 188U, from 

 its discovery by Mr. John Young in the Carboniferous species, S. Uneatus, 

 Martin. It had been observed by us some years earlier than this in the species 

 S. fimbriatus, Conrad, of the Hamilton group, and upon the accompanying 

 plates are illustrations of this species showing it. The scar left by the removal 

 of the spines is elongate-oval or lachrymiform, divided medially by the base 

 of the vertical lamella. In addition to this surface ornament the members of 

 this subdivision are well characterized by their shortness of hinge, which does 

 not equal the greatest transverse diameter of the shell, the low cardinal area, 

 the usual obsolesence or absence of radial plications and even of median fold 

 and sinus, and the presence of concentric striations or ridges serving as bases 

 for the fimbriae. 



Of this group Spirifer fimbriatus, Conrad, is the earliest representative, a 

 form quite remarkable in its vertical range, appearing in the Oriskany sand- 

 stone and in the succeeding faunas of the Schoharie grit, Corniferous limestone, 

 Hamilton shales and Chemung sandstones. This species and S. subundiferus. 

 Meek and Worthen, of Hamilton age, have more of the typical spiriferoid 

 aspect in their well developed fold and sinus and low plications ; in these 

 respects they afford a variation from the later members of the group, which is 

 naturally to be expected in the earliest representatives of a type of structure. 

 The group is, on the whole, possessed of very persistent and stable characters, 



