386 NORTH AMERICAN INDEX FOSSILS. 



XVIL Pal^ocaris Meek and Worthen. 



Shrimp-like, with the two pairs of antennae of nearly equal length, 

 inner biramous ; head as long as first two abdominal segments ; 

 thoracic legs long, slender, anterior pair without chelae; telson 

 long and flat; last pairs of pleopoda flattened, with short first joint. 

 Carbonic. 



30. P. typus Meek and Worthen. (Fig. 1691, c-e.) Carbonic. 



Thorax slightly wider in middle than the abdomen ; telson nearly 

 as broad at base as penultinate segment ; both telson and stylets 

 setaceous. (Type of genus.) 



Coal Measures of Illinois. 



Order DECAPODA Latreille. 



Crustacea with the cephalon and thorax united into a cephalo- 

 thorax, of thirteen segments, each with a pair of appendages, and 

 the whole completely covered by a single carapace, or with one 

 segment free. Anteriorly the carapace is commonly prolonged into 

 a median spine or rostrum, which may be continued backward in 

 a median dorsal ridge or keel. The surface of the carapace is com- 

 monly divided by grooves or depressions into a number of regions, 

 corresponding in a general way to the grouping of the organs lying 

 below it. A transverse neck furrozu {cervical sulcus) generally 

 divides the carapace into " cephalic " and " scapular " region, the 

 latter being commonly the larger. The anterior region is divided 

 by vertical or oblique furrows into a median gastric region, and 

 lateral hepatic regions. The posterior region is similarly divided 

 into the median cardiac region, and the lateral branchial regions. 

 The grooves vary greatly, and are often obsolete. The anterior 

 pair has been designated the gastro-hepatic grooves, and the pos- 

 terior pair as the branchio-cardiac grooves. 



The ventral surface of the carapace commonly shows a more or 

 less well-developed sternum, which occupies the inner field between 

 the thoracic legs, and varies in width according to the distance be- 

 tween the inner leg-bases. It is especially well developed in the 

 Brachiura, where it shows more or less strongly, the original seven- 

 partite structure. 



The abdomen (post-abdomen) is free and distinct though not 



