388 NORTH AMERICAN INDEX FOSSILS. 



five pairs (hence the name Decapoda), and with few exceptions 

 each consists of seven parts or joints. The exopodite is absent or 

 rudimentary, the leg consisting of the two joints of the protopo- 

 dite, and the five- jointed endopodite. Named from the base out- 

 ward, the joints are: i, coxopodite {coxus) ) 2, basipodite {tro- 

 chanter primus) ; 3, ischiopodite (trochanter secundiis) ; 4, meropo- 

 dite (merus) ; 5, carpopodite {carpus) ; 6, propodite {hand, 

 manus) ; and 7, dactylopodite {free finger, dactylus pollex). The 

 first two are generally short, while the last two often constitute the 

 shears or chela. The carpus may be many jointed (Fig. 1692) . The 

 chelae are generally best developed on the first pair of thoracic legs 

 {chelopods), which are commonly larger, though often unequal in 

 the two legs. The succeeding legs are generally claw-like and 

 serve for walking purposes, though one or more pairs may be 

 modified into flat paddles for swimming purposes. Not infre- 

 quently, however, one or more pairs of the succeeding legs may be 

 chelate, though these are commonly much smaller than the anterior. 



The abdomen is typically furnished with six pairs of abdominal 

 legs {pleopoda), corresponding to the first six segments. Each 

 consists of a two-jointed stem or protopodite, and two branches 

 (exopodite and endopodite). In the Brachiura generally only 

 a few of the pleopoda are present. In the Macrura the exopodite 

 and endopodite of the sixth segment are leaf-like and flat, and 

 form with the telson the caudal fin. 



Typical Decapod Crustacea appear first in the Triassic, are not 

 uncommon in the Jurassic and Cretacic, and abound in the Ter- 

 tiary and modern faunas. American fossil forms are known from 

 the Cretacic and the Tertiaries. 



Literature. 

 1863. Stimpson, W. On the Fossil Crab of Gay Head. Boston 



Journal of Natural History, Vol. VH., pp. 583-589, pi. XII. 

 1870. Cope, E. On Three Extinct Astaci from Idaho. Proc. Amer. 



Phil. Soc, 1869-1870, p. 605. 

 1 880- 1 88 r. Packard, A. Fossil Crayfish from the Tertiaries of 



Wyoming. Am. Naturalist, Vol. XIV., p. 222; Vol. XV., pp. 832 



-834, and figure. 

 1903. Whiteaves, J. F. Mesozoic Fossils, Part V. Crustacea, 



pp. 315-326, pi. XL.-XLI. 



