516 A POPULAR EXPOSITION OF THE 
but their specific characters are necessarily somewhat obscure. Finally, in the 
genus Ateleocystites, a single species, A. Hualeyi, is mentioned. The calyx in 
this form appears to have, as in Pleurocystites, a dorsal side made up of com- 
paratively few plates, with numerous small plates on the ventral side. In other 
respects, however, the genus is a very peculiar one, aud perhaps referable to a 
distinct group.” 
4. Thyroida.—This Order is represented by a single genus, Agela- 
crinites: a peculiar type, connecting the cystideans with the star- 
fishes. It presents a somewhat flat, circular form with a five-rayed 
ambulacral star at the upper part, composed each of two series of 
interlocking plates, with the intermediate spaces covered by numerous 
scale-like imbricating plates, arranged more or less irregularly. The 
rays in some species are long and curved, 
and in others straight and short. Between 
two of these rays there is a circular open- 
ing, covered by five or ten triangular plates 
in the form of a “pyramid,” as in the 
cystideans. The species range from the 
Lower Silurian to the Carboniferous forma- 
tions. Figure 86 represents Agelacrinites 
Billingsii of the Trenton limestone (Lower 
Silurian). Other species from the same 
formation, A. Dicksoni and A. (Hdrioaster) Bigsbyi, have long curved 
rays. (Decade III. “Canadian Organic Remains.” See also for a 
more complete description of A. Billingsii, a paper by the writer in 
the Canadian Journal, Vol. V., p. 350, and in the Annals of Natural 
History, August, 1860.) 
5. Asterida.—This Order includes the greater number of the so- 
called star-fishes. The body is covered by a thick skin, strengthened 
by plates and tubercles of carbonate of lime. There is no stem, and 
the mouth is always on the underside of the body, in the centre of the 
arms or rays. These are five or more in number. The visceral 
cavity or stomach extends into them. Species occur in all formations 
from the Lower Silurian upwards, but the Order appears to be more 
numerous in existing seas than in the waters of any former epoch. In 
the Third Decade of Canadian Organic Remains, Mr. Billings describes 
several species from the Lower Silurian rocks. These are placed 
under the following genera, but specimens, it should be observed, are 
of rare occurrence, and the characters of those obtained are still more 
or less obscure. 
E.d.€. Neb, ELLIS 5c 
Fig. 86. 
