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Field Columbian Museum — Geology, Vol. II. 



1 878 by Angelin* to receive a number of species from the Silurian rocks 

 of Gotland, Sweden. It has been recognized in the Wenlock limestone 

 in England, and Wachsmuth and Springer described two species from 

 Indiana which they referred to the subgenus Acacocrinus. The differ- 

 ences between Habrocrinus and Pionocrinus are as follows: In Hab- 

 rocrinus the interbrachial areas consist of one plate resting on the 

 superior lateral edges of the radials, followed by two plates in each 

 of the succeeding rows; in Pionocrinus the first interbrachial plate 

 is large, occupying the space between the two rays from the superior 

 lateral edges of the radials to the inferior edges of the first distichals. 

 This plate is followed by a single narrow plate. The first costals in 

 Habrocrinus are hexangular, while those of Pionocrinus are quadran- 

 gular. These differences are not considered to be of generic import- 

 ance, so that Bather and Wachsmuth & Springer place the species 

 originally referred to Pionocrinus in the genus Habrocrinus. 



Fig. 9. Outline of Habrocrinus ornattis Ang. 

 Posterior Views. (Alter Angelin). 



Lateral and 



Fig. 10. Outline of Piono- 

 crinus jarctus Ang. Lateral 

 View. (After Angelin). 



Habrocrinus differs from Periechocrinus in character and ornamen- 

 tation of the plates of the dorsal cup ; in the former the plates are thick, 

 more or less sculptured, sutures situated in deep furrows; in the 

 latter the plates are thin, edges not beveled and position of sutures 

 not well defined. In Habrocrinus the arms are strong and simple; 

 in Periechocrinus they are slender and branching. In the former 

 the palmers are not present in the dorsal cup ; in the latter they are 

 present. In the former the number of arms is two to each ray, in the 

 latter it is four or more to each ray. 



Of the fourteen American species referred to Periechocrinus , four 

 species, P. benedicti S. A. M., P. chicagoensis Weller, P. howardi S. A. M. 

 and P. ornatus Hall, possess characters which appear to necessitate 

 their removal from that genus to Habrocrinus. The characters referred 

 to are as follows : The absence of the palmers in the dorsal cup, reduc- 

 ing the number of arm bases from four to two in each ray, and the 

 * Iconographia crinoideorum, p. 3. 



