42 PALEONTOLOGY OF NEW YORK. 



Spirifer without the introduction into th.at genus of an inconvenient and 

 refractory element. The fact of their variation in ornamentation while re- 

 taining the same contour, prevents the assimilation of the group into any of 

 the subdivisions of Spirifer proposed above, and for such a reason it will be 

 well to recognize the term, since it has been so long in use. 



Cyrtia is, in effect, a group of cyrtiniform Spirifers, and its specific repre- 

 sentation is quite small. The type species C exporreda, Wahlenberg, occurs in 

 the Wenlock limestone, and in the fauna of the Niagara group the same form 

 is associated with the variety arreda, Hall and Whitfield. Mr. Billings's species 

 C. Mydea, from Division 4 of the Anticosti series, appears to be identical with 

 C. exporreda. These, and a larger form from the Niagara dolomites of Wiscon- 

 sin, which we shall term C. radians, sp. nov., all have the filamentous surface 

 markings which characterize the Silurian Spirifers of the S. radiatus-type, 

 though they show no tendency to become plicated. In the Devonian faunas 

 the external ornament changes ; thus in C. Murchisoniana, the shell is finely 

 plicated on the sides and over fold and sinus, as in the Spirifer disjundus-group 

 of the Ostiolati ; in C. cyrtiniformis. Hall and Whitfield, of the upper Devonian 

 of Iowa, the plications are coarser and more nearly equal over the lateral and 

 median regions; in C. simplex {Spirifer simplex, Phillips), of the middle Devonian 

 of Great Britain and Europe, the surface, as usually preserved, is apparently 

 smooth, with sometimes traces of a few coarse lateral plications near the mar- 

 gins. Finely preserved examples of this species from the vicinity of Bredelar, 

 Westphalia, show that the surface is covered with closely crowded concentric 

 rows of very fine and short, simple spinules, as in the unicispinate group of the 

 fimbriate Spirifers. The Spirifer alius, Hall, of the Chemung group, is another 

 form which may be referred to Cyrtia. It has the lateral slopes more strongly 

 plicated than C. simplex, and traces of plications are also visible upon the fold 

 and sinus. It is remarkable for its great size and also for its agreement with 

 C. simplex in the peculiar retrorse slope of the cardinal area which throws the 

 apex of the pedicle-valve over, or in front of the center of the shell. In the 

 Devonian Cyrtias the foramen in the deltidium is frequently obscured or absent 

 at maturity. It may have existed at earlier stages of development and have 



