some British Fossil Crustacea. 393 
Char. Oval, moderately convex; head semicircular, the angles 
rounded, bearing two large oval or slightly reniform glo- 
merated masses of minute round eyes ; thoracic segments seven, 
broad, shghtly granulated, with obtusely rounded ends, each 
extremity having a long triangular facet on its anterior mar- 
gin (to facilitate rolling into a ball) ; abdomen of five segments, 
the first three abruptly smaller than the thoracic rings, the 
fourth a little larger, and the fifth forming a semicircular 
caudal shield, rather smaller and more convex than the head, 
bearing along its middle a narrow, defined, semicylindrical axal 
lobe, its rounded termination not reaching much more than 
halfway to the margin, the anterior end extending a variable 
distance towards the thorax. 
I have not seen any trace (after examining about fifty speci- 
mens) cf the lateral notches in the caudal shield for the articu- 
lation of lateral appendages, which Dr. Milne-Edwards says he 
thinks he saw. The only known species averages 6 lines long 
and 32 lines wide. 
(Col. University of Cambridge.) 
Ord. Enromosrraca. 
(Trib. Pecilopoda.) 
This group being distmguished from other Entomostraca by 
having crustaceous, didactyle, ambulatory thoracic feet as well as 
membranous, respiratory abdominal ones, is I think clearly the 
place for those remarkable genera, Hurypterus and Pterygotus ; 
{ cannot conceive why Dr. Burmeister should imagine the first 
of those genera to have no shell, and overlooking the didactyle 
structure of the larger crustaceous chelz, &c., place it in his group 
Paleade (Dal.), which, as he observes (Organiz. Trilob., Ray ed. 
p- 53), might be united with the Phyllopoda. The figure and 
description given by Romer of the American species of Hury- 
pterus in his paper in Dunker and Von Meyer’s ‘ Beitrige zur 
Naturgeschichte der Vorwelt,’ powerfully favour this view of ap- 
proximating the genus to Limulus. With regard to the second 
genus, Pterygotus, M. Agassiz having renounced his original opi- 
nion of its being a fish, has, in his work on the Fishes of the Old 
Red Sandstone, referred it to the Entomostraca without indica- 
ting any particular division. Some years before the appearance 
of the ‘ Poissons fossiles des vieux grés rouge,’ I had an oppor- 
tunity of examining some much more perfect examples of this 
Crustacean than are there figured, which were brought before the 
Geological Society of Dublin by Dr. Scouler under the name 
Lepidocaris (from the scale-like sculpturing of the cephalic 
shield) *, and except the enormous difference in size, and perhaps 
* See Dr. Apjohn’s President's Address. 
