MUSEUM OF COMPARATIVE ZOOLOGY. 263 



internal border of the segment (turned away from the reader in the 

 figure) are not duplicated on the homologous margin of the upper half 

 of the segment. Articulated with the distal end of this segment are 

 two carpi (4, 4'). The supernumerary carpus (4') does not have the 

 exact form of the normal carpus (4), but is slenderer, subcylindrical, and 

 much more spiny. The normal carpus is followed by a propodus and 

 dactylus (5, 6) of the regular form. The supernumerary carpus bears 

 at its distal extremity an abortive propodus (5') in the shape of a small 

 stump-like segment, bifurcated at the end and armed with a blunt 

 spinous tubercle (,/) on its inner margin. This tubercle is homologous 

 with the tubercle y at the proximal end of the external border of the 

 normal propodus. Curiously, the supernumerary carpus is set upon the 

 meros in a position almost the reverse of that of the normal carpus, so 

 that the surface of the accessory carpus and propodus, which is homolo- 

 gous with the upper surface of the regular carpus and propodus, looks 

 in almost the opposite direction. It is as if the abnormal carpus were 

 rotated upon the meros through nearly 180 to the left. It thus comes 

 about that the articular tubercle x' falls on the same side with its 

 homotype, x, instead of on the opposite side, as one would expect from 

 the reversed symmetry of the two carpi. If the two propodal segments 

 (5, 5') were flexed at the same time, they would move in nearly opposite 

 directions. This distortion seems to me very singular, and I think 

 nothing like it has been observed among the many cases of double legs 

 in insects. 



In this specimen we have the nearest approach to complete duplica- 

 tion of a limb yet observed among Crustacea. It reminds one of the 

 monstrosities among insects, frequently described by entomologists, in 

 which the duplication of a leg may involve all the joints down to the 

 trochanter. Whether this monstrosity be congenital, or the result of 

 injuries received later in life, I cannot tell. 



Plate II. Fig. 7 (Homarus Americanus, left chela). — In this small 

 chela only a rudiment of the index is present, and the dactylus is curled 

 underneath it in the form of a semicircle. 



Plate II. Fig. 8 (Callinectes hastatus, left lateral portion of the cara- 

 pace). — The lateral horn, instead of being simple, as in normal speci- 

 mens, has three spines, one directed forward, outward, and downward, 

 one backward, outward, and upward, and one, very small in size, back- 

 ward, outward, and downward. 



Plate II. Fig. 9 (Homarus Americanus, right chela). — The whole of 

 the index as well as part of the hand is wanting in this sadly mutilated 



