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 cheliped figured on Plate II. (Fig. 6) 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, 16, and in Plate II. fig. 2.* Jäger (No. 10, p. 38, figs. 12, 13) has 
described and figured a claw of Uca una with two dactyli, and a similar 
case in Zriphia spinifrons has been published by Herklots (No. 15, figs. 
6, 74 8). 
On seeing such a specimen as the Prionus figured by Jayne,} in which 
the tibiæ 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 may be referred to a vitium primce 
conformationis, and in examples from Crustacea such as those represented 
by Fig. 13, Plate L, and Fig. 6, Plate IT., of this paper, it is very probable 
that we are dealing with a monstrosity which is not the result of in- 
jury. 
Most of the deformities by 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 super- 
numerary daetylus, we find that it is commonly developed from the 
normal dactylus, and devoid of any articulation, instead of joining by an 
articular surface with the propodus as a true supernumerary dactyle 
would do. 
In this category the Astaci noticed by Emmanuel Rousseau (No. 8) and 
Eugéne Desmarest (No. 9) will also be included. In these abnormal 
female specimens an extra pair of vulva were present on the basal seg- 
ment of the fourth pair of legs, the oviduot 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 
Peabody Aeademy of Science, Salem. 
1 Op. cit., Plate IV. fig. 12. 
