22 TRUE CRABS OF MONTEREY BAY 



taken in the construction of keys, as this family usually proves trouble- 

 some to the novice, chiefly, it would seem, because of the characters 

 made use of in keys, which are commonly, and almost necessarily, in- 

 accessible or indefinite. The degree of completeness of the orbit, though 

 undoubtedly one of the most obvious of certain modifications involving 

 the greater part of the head region, is generally used in a comparative 

 manner as "more" or "less complete," which except to the person familiar 

 with the forms is little better than useless. When present the orbit seems 

 to be formed by the development of the pre- and post-ocular spines and 

 by the expansion of the basal antennal joint. Of these the 

 post-ocular spine is the most important; absent or distant from the 

 eye in forms like Podochela and Erileptus, it becomes, in Chionoecetes 

 and allied species, cup-shaped, and into this cavity the eye is retracted. 

 I have attempted to make use of this character in the generic key, which 

 follows in part that of Miss Rathbun on this group.* A second key 

 based on more obvious and accessible characters and including the species 

 is given as an additional aid. 



Some interesting correlations were noted in the series, which though 

 probably not unknown, I have not seen pointed out before. The length 

 of the rostrum is correlated, not only, as might be expected, with the 

 length of the carapace, but also with the length of the antennae. The 

 flagellum is usually somewhat shorter than the rostrum and corresponds 

 quite closely to it in length ; this is especially noticeable in comparing a 

 form with a long slender rostrum as Chorilia with one having a short 

 rostrum, as Chionoecetes. In a few cases (Mimulus) the antennae exceed 

 the rostrum. As follows from the length, the antennae correspond 

 in stoutness with the rostrum ; but in addition, those species hav- 

 ing markedly flattened rostrums are found to have flattened antennae 

 (Scyra, Pugettia) while those with long cylindrical horns forming the 

 rostrum have cylindrical antennae (Chorilia). 



I have followed Miss Rathbun in ignoring, as in the report on the 

 Decapods of the Harriman Expedition, the subfamilies of this group, 

 which seem, at least on this coast, to be of little service and incapable of 

 sharp distinction. 



*Am. Nat., xxxiv, 503-520, 1900. 



