146 University of California Publications in Zoology [Vol. 23 



Parapagurus mertensii (Brandt) 



Plate 16, figure 5 



Pagurus mertensii Brandt, in Middendorff, Eeise in den aussersten Norden 



und Osten Sibiriens, Bd. II, Zool., Th. I, p. 112, 1851. 

 Parapagurus mertensii Holmes, Oceas. Papers Calif. Acad. Sei., 7, 155, 

 1900 ; Eathbun, H. A. E., 10, 162, pi. 5, fig. 6, 1904. 



Characters. — Median projection of front prominent, somewhat elongate, tip 

 rounded, sides subparallel, lateral projections small; anterior portion of carapace 

 about as wide as long. Eye-stalks short, about one-half length of anterior portion 

 of carapace. Chelipeds pubescent, spiny, very unequal; right cheliped very large, 

 carpus long, inner and outer margins spiny, convex upper surface armed with 

 two rows of short spines, hand long, narrow, dorso-ventrally flattened, and bent 

 inward at a slight angle to the carpus, rounded upper surface with small, sub- 

 serially arranged granules which become more prominent on the fingers, inner 

 and outer margins sharp, granulo -denticulate and parallel; left cheliped long, 

 slender and attenuate, carpus subcylindrical, with three rows of spines on the 

 upper surface, hand narrow, much longer than the carpus, palm very short, fingers 

 long, narrow, and curved downward. Dactyls of ambulatory legs exceedingly long 

 and slender, armed below with numerous spines. 



Dimensions. — Type: length of carapace 19.1 mm., width 14.8 mm. 



Type Locality. — Kamchatka. 



Distribution. — Kadiak Island, Alaska, to off San Nicolas Island, California, 

 77 to 266 fathoms. 



Remarks. — This species affords one of those curious cases of commensalism with 

 colonies of hydroids sometimes found among deep sea pagurids. The colony of 

 hydroids covering the shell in which the crab lives forms a membranous growth 

 which extends beyond the boundary of the shell and in course of time, according 

 to Dr. Benedict, causes the shell to disappear, leaving its inhabitant with a mem- 

 branous instead of a calcareous domicile. In the specimen which I have the 

 covering is partly formed by the tip of a broken shell but mainly by an extension 

 of the membranous growth formed by the colony of hydroids. This arrangement 

 certainly affords the crab the advantage of allowing it to grow without having 

 to undergo the troublesome experience of changing lodgings (Holmes). 



Family Lithodidae 



Body crab-like. Abdomen more or less firm, in part at least; often segmented, 

 bent under thorax; no tail-fan developed, uropods absent. Carapace quite firm 

 all over. First pair of legs chelate; fourth pair like the third; fifth pair very 

 small and folded under the carapace, making it appear superficially that the 

 members of this family have but four pairs of legs, a character which easily 

 distinguishes it from any other family of Decapods. 



Key to the California Genera of the Lithodidae 



I. Abdomen soft, more or less loosely inflexed, unsegmented ; at most only basal 

 (second), and two terminal segments stiffened by thin calcareous plates; 

 reinforcement of basal segment usually consisting of a pair of narrow 

 marginal plates and a pair of lateral plates, between which there may be 

 a median plate; abdomen occasionally armed with calcified granules or 

 short spines. 



