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, slightly 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) of 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 3^ lines wide. 



(Col. University of Cambridge.) 



Ord. Entomostraca. 



(Trib. Pcecilopoda.) 



This group being distinguished 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, Eurypterus and Pterygotus ; 

 I 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 chelse, &c, place it in his group 

 Palceadce (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 Eury- 

 pterus in his paper in Dunker and Von Meyer's ' Beitrage 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 gres 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. 



