140 CRUSTACEA. 



membranous, respiratory abdominal ones, is I think clearly the 

 place for those remarkable genera, Euryptervs 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 chelae, &c., place it in his group 

 Pal(Bad(2 (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 difi'erence in size, and perhaps 

 a difi'erence of superficial sculpturing, I see nothing in it different 

 from Eurypterus ; and when we bear in mind that the Idothea of 

 Scoulert is avowedly a Eurypterus, I cannot see how Pterygotus 

 is to be separated as a genus, at least on any better grounds 

 than the above. The tribe Pcecilopoda might be resolved into 

 two families : 1st, Limulidce, having, besides the head, a second 

 shield formed by the anchylosis of all the abdominal segments 

 {Limulus) ; 2nd, Eurypterid(S, having all the abdominal segments 

 distinctly separated [Eurypterus, Pterygotus, Bellinurus). The 

 first division has not, 1 believe, been found lower than the oolites, 

 the Limuli quoted by several British geologists from the coal- 

 measures of Coalbrook Dale, &c. belonging clearly to the second 

 division, and should rather be referred to Bellinurus of Konig. 



Pterygotus leptodactylus (M'Coy). 

 Sp. Char, Large pincers having the hand about 5 lines wide, 

 sculptured with fine short, irregularly flexuous, elevated lines ; 

 the penultimate or immoveable finger exceedingly slender, 

 compressed, about 2 inches 10 lines long, and only 2 lines wide 

 at base, gradually tapering to less than a line tov\^ards its ob- 

 tuse point, nearly straight, or with a scarcely perceptible in- 

 ward curvature ; sides divided into ridges by three or four 

 longitudinal furrows, thicker towards the back; last joint 

 or moveable finger similar to the immoveable one, but rather 



• See Dr. Apjohn's President's Address, 

 f See Edinb. Journal of Science, vol. iii. 



