294 FIELD COLUMBIAN MusEuM — GEo.oey, VoL. II. 
1878 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 Ptonocrinus 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 Ptonocrinus in the genus Habrocrinus. 

Fig. 9. Outline of Habrocrinus ornatus Ang. Lateral and Fig.10, Outline of Piono- 
Posterior Views. (After Angelin). crinus farctus Ang. Lateral 
View. (After Angelin). 
Habrocrinus differs from Pertechocrinus 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 Pertechocrinus 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 Pertechocrinus, four 
species, P. benedict S. A. M., P. chicagoensis Weller, P. howard1S. 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. 
