MUSEUM OF COMPARATIVE ZOOLOGY. 265 



from the zoea to the crab in the species described in this paper, and that he 

 had, besides, another zoea, apparently distinguished from mine only by its 

 slightly smaller size and shorter spines, which passed into a megalopa phase ! 

 In this stage (a tracing of which is enclosed in Professor Smith's letter) the 

 carapace is broad and crab-like, without a vestige of the spines of the zoea. 

 This megalopa subsequently changed into a young crab very nearly like that 

 which developed immediately from the long-spined zoea.* 



It would seem, therefore, that two species of Pinnixa occur, at least in the 

 young stages, on the southern shore of New England, which present a remark- 

 able difference in their development. While the one goes through the custom- 

 ary megalopa condition, the other passes, by a syncopated development, from 

 the zoea directly into the brachyurous state. 



Cambridge, January, 1879. 



EXPLANATION OF THE PLATES. 



Note. I am greatly indebted to my brother, Mr. Charles E. Faxon, for preparing my drawings 

 for the heliotypic process. 



PLATE I. Hippa talpoida. 



Fig. 1. Cluster of segmented eggs taken from the abdominal appendages of a fe- 

 male. The eggs are spherical, about 1.5 mm. in diameter, of a bright 

 orange-yellow color. 



Fig. 2. Egg with the enclosed embryo in the " nauplius-stage " : a, labrum ; b, 

 antenna of the first pair ; c, antenna of the second pair ; d, mandible ; 

 e, abdomen ; /, yelk. 



Fig. 3. Embryo further advanced : a, rostrum ; b, cephalic plate ; c, antenna of 

 first pair ; d, antenna of second pair ; e, mandible ; /, first maxilla ; g, 

 second maxilla ; h, first maxilliped ; i, second maxilliped (even at this 

 early stage, the appendages back of the mandible are double) ; k, abdo- 

 men (the likeness of the abdomen in this embryonic stage to the abdomen 

 of prawn-zoese is suggestive). 



Fig. 4. The same, from a different point of view : a, antenna of first pair ; b, an- 

 tenna of second pair ; c, mandible ; d, first maxilla ; e, second maxilla ; 

 /, first maxilliped ; g, second maxilliped ; h, hind border of carapace. 



* Professor Smith has also kindly sent me tracings of the zoea of Pinnotheres 

 maculatus raised from eggs. It has a long dorsal spine (wholly wanting in Thomp- 

 son's figures of the zoea of Pinnotheres pisum), and the abdomen is like that figured 

 by Fritz Muller as Pinnotheres, "Fur Darwin," Fig. 19 (v. supra, p. 263, notef). Al- 

 together the zoea bears less resemblance to the zoea of Pinnotheres pisum, as represented 

 by Thompson, than the zoea of Pinnixa does ! It should be noted that the accuracy 

 of Thompson's figures is attested by Bell (" British Stalk-eyed Crustacea," p. 125). 



