350 NEW YORK STATE MUSEUM 
stration of the identical structure of the compound eye of Pterygotus 
with that of Limulus and the determination of the endostoma. 
The characteristics of this genus consist in the lack of distinct dif- 
ferentiation of the body into preabdomen and postabdomen; the large 
size of the compound eyes, their marginal position and distinct facets; 
the presence of a distinct epistoma, the enormous development of the 
preoral, chelate appendages, the slenderness and lack of spines on the 
four pairs of walking legs, the heart-shaped, deeply emarginate metastoma 
and the broad telson. 3 | 
Huxley and Salter placed P. bilobus in a separate subgenus, 
Erettopterus, on account of its bilobed telson. While some have recog- 
nized this group as a distinct genus, others have not given it even sub- 
generic rank. Laurie states that the species with such telsons “ might 
fairly be separated from the rest as a subgenus, were it not that the frequent 
absence of the tail would make such an arrangement highly inconvenient.”’ 
In our view the recognition of a natural group can not be a matter of 
convenience, but is of necessity, when the differences are recognized as 
subgeneric in rank. Erettopterus is represented in our Siluric rocks by 
P. grandis of the Bertie waterlime, P. globicepsof the Shawangunk 
grit and certain telsons from the Frankfort shale. 
The presence of Pterygotus in the North American Devonic has been 
shown by Logan, Billings and Clarke, in the Grande Gréve limestone 
(Lower Devonic), by Dawson in the Gaspé sandstone, by Clarke in the 
Dalhousie formation and by Whiteaves in the Upper (?) Devonic at 
Campbellton, All these remainsfrom Eastern Canada are fragmentary and. 
only those of Campbellton have justified description (P. atlanticus). 
Pterygotus macrophthalmus Hall 
Plate 69, figures 2-7; plate 70, figures 1, 2; plate 71 
Pterygotus macrophthalmus Hall. Palaeontology of New York. 1859, 
3: 418*, pl. 80A, fig. 8, 8a 
Undet. crustacean. Ibid. pl. 80A, fig. 6 
P.osborni Hall. Jbzd. p. 419*, pl. 80A, fig. 9 
Description. Cephalothorax. Outline of carapace or cephalothoracic 
. shield semielliptic. In the type of the species, the carapace of a young 
