MUSEUM OF COMPARATIVE ZOOLOGY. 267 



involving only the distal segments. In the leg, for instance, all the seg- 

 ments beyond the coxa may be duplicated, while in other cases one or 

 two of the distal joints of the tarsi alone will be repeated. 



Among Crustacea the examples of a real duplication or triplication 

 of segments in an appendage are very rare. The most marked instance of 

 the kind is afforded by the lobster chelipcd figured on Plate II. (Fig. G) of 

 this paper, in which there is a clear tendency to duplication, at least from 

 the coxa onward. Duplication of the dactylus is seen in Plate I. figs. 12, 

 13, IG, and in Plate II. fig. 2.* Jager (No. 10, p. 38, figs. 12, 13) has 

 described and figured a claw of Uca una with two dactyli, and a similar 

 case in Eriplda spinifrons has been published by Herklots (No. 15, figs. 

 6, 7, 8). 



On seeing such a specimen as the Priomis figured by Jayne,f in which 

 the tibiae and tarsi are duplicated in all the legs, and perfectly symmetrical 

 on the two sides of the body, one cannot doubt that in insects at least 

 these monstrosities by duplication mtiy be referred to a vitium j^rimx 

 conformationis, and in examples from Crustacea such as those represented 

 by Fig. 13, Plate I., and Fig. G, Plate II., of this papei', it is very probable 

 that we are dealing with a monstrosity which is not the result of iu- 

 jtiry. 



Most of the deformities hy excess among Crustacea, however, do not 

 result from a true duplication of more or fewer segments of an appendage, 

 but from the outgrowth of unarticulated processes of various shapes, 

 often furnished with teeth, and simulating true segments. But in 

 such cases, e. g. where there is a process that has the form of a supei"- 

 numerary dactylus, we find that it is commonly developed from the 

 normal dactylus, and devoid of any articulation, instead of joining by an 

 articular surfiice with the propodus as a true supernumerary dactyle 

 would do. 



In this category the Astaci noticed by Emmanuel Rousseau (No. 8) and 

 Eugene Desmarest (No. 9) will also be included. In these abnormal 

 female specimens an extra pair of vulvae were present on the basal seg- 

 ment of the fourth pair of legs, the oviduct of each side dividing into 

 two branches after leaving the ovary. 



c. Monstrosities of this class result from an organ being replaced 

 wholly or in part by another organ. Such monstrosities are common in 

 plants, but exceedingly rare in animals. A few have been described 



* There are two or three lobster claws with two dactyli in the collection of the 

 Peahody Academy of Science, Salem, 

 t Op. cit., Plate IV. fig. 12. 



