450 
Aug. F. Foerste 
sense that they can not be located readily in specimens which have 
their thecal plates more or less disarranged, and this is true of the 
types of Agelacrinus warrenensis. 
An excellent young specimen of Agelacrinus cincinnatiensis 
is preserved in the American Museum of Natural History, and 
forms No. 1194-TV of that collection. It is 10 mm. in diameter, 
rests upon a Rafinesquina, and displays five rays, one of them 
dextral. The tips of the rays already are sufficiently curved to 
become parallel to the peripheral band. An examination of this 
specimen will readily illustrate how difficult it would be to re cog- 
nize the rays if the thecal plates were only moderately disarranged 
while the disarrangement of the thecal plates of the types of Agel- 
acrinus warrenensis was fairly considerable. 
36. Streptaster, Hall, generic characteristics 
The term Streptaster was proposed by Hall in 1872 as a subgen- 
eric term under Agelacrinus, and was founded on Streptaster vorti- 
cellatus, Hall, at that time the only species known. Later, in 
1878, a closely related species, Streptaster septemhrachiatus, was 
described by Aliller and Dyer. Since both species had all of the 
rays sinistral, this appeared to be the most striking characteristic 
of the genus. In the present paper, however, a species with the 
right posterior ray turned in a dextral direction is described, so that 
this feature looses in diagnostic value. . 
The chief characteristics of Streptaster, as far as known at pres- 
ent are the following. 
Interambulacral areas very narrow, composed of a mosaic of small 
polygonal plates. Ambulacral rays very prominent and narrow, strongly 
curved, consisting of long, linear, lateral covering plates suggesting verti- 
cal palisades on lateral view, and enclosing high but very narrow ambu- 
lacral spaces. Floor plates small, each floor plate supporting two cover- 
ing plates, one on each side. These floor plates overlap strongly in a 
proximal direction, and are so small that the bases of the covering plates 
must be almost in contact with each other. Peripheral band of plates 
distinctly defined, with larger, wide plates toward the top of the band, 
and with smaller, scale-like plates toward the margin, as in Ordovician 
species referred to Agelacrinus and Lepidodiscus. 
