19 



portion of the joint is less clearl}^ shown bj an elevated, dis- 

 colored portion of the rock, showing here and there a trace of 

 the test. In form and size it very nearly agrees with the same 

 part of P. anglicus Ag., from the Devonian of Forfarshire, fig- 

 iired by Woodward (Mon. of British Fossil Merostomata, PL 4). 

 So far as can be seen it differs from that figure less than do some 

 of the other specimens of that species, figured in the same mon- 

 ograph, and if it occurred in the same bed, little doubt could be 

 entertained of its identity. This part, however, does not appear 

 to afford good specific characters since the figures of different 

 specimens show considerable variation. The specimen of P. 

 Cummingsi before us appears to have belonged to as large an 

 individual as any known of P. anglicus^ which Woodward esti- 

 mates to have attained a length of about six feet, and a breadth 

 of about two feet. Although apparently agreeing so closely with 

 P. anglicus^ its identity cannot be sustained, from the fact that 

 P. anglicus is found in a higher horizon. 



Three species of this genus have been described from the 

 American Water Lime Group, from fragments of different parts 

 of the body. As no two of these are founded on the same part, 

 no comparison, except that of size, is possible. P. macrocephalus 

 Hall, and P. Osborni Hall, are evidently very much smaller 

 species, measuring scarcely as many inches as P. Cwmmingsi 

 does feet, and are undoubtedly distinct. The third, P. Cobbi 

 Hall, is based on "the free ramus of a chelate appendage." 

 Judging from the proportions of the corresponding parts of P. 

 anglicus to tfce entire size, Prof. Hall's specimen belonged to an 

 individual from 1^ feet to 2 feet shorter than P. Cummingsi. 

 More complete specimens are needed to establish the relations of 

 these two forms, but for the present it seems advisable to regard 

 them as distinct. 



The maxilliped of Pterygotus Cummingsi has a breadth of 

 three and a half inches. 



