1878.] AND DISTRIBUTION OF THE CRAYFISHES. "69 



the ends, and otherwise similar in structure to those of Astacopsis, 

 are attached to the posterior edge of the stem of the podobranchia 

 (fig. 6, vu). 



The ala is widest, and extends furthest towards the summit of the 

 branchia, in the most anterior podobranchia, while in the hinder- 

 most it is reduced to little more than a longitudinal ridge. This 

 branchia, in fact, is very similar to one of those of Astacopsis. 



There are six anterior arthrobranchiae, which are not more than 

 half as long as the podobranchiae, and five still smaller posterior ar- 

 throbranchise, the hindermost of which is almost rudimentary. 



As in Astacopsis, there are four large pleurobranchiae, the hinder- 

 most of which is only slightly larger than the other three, which are 

 nearly equal in size. 



Small branchial filaments are scattered over the whole posterior 

 face of the epipodite of the first maxillipede ; and the apex of each 

 is provided with a strong sickle-shaped hook. 



The coxopoditic setae are relatively short, as in Astacopsis, but 

 slender and curved, and even slightly undulating (fig. 6, vi). Each 

 is hooked at its free extremity (fig. b', v). 



If the distinctive character of Chceraps, as given by Erichson, is 

 correct, this is not a true Chceraps ; but the branchia? of these 

 animals are so readily detached, that I can give no opinion on this 

 point without comparison with the original specimens. In any case, 

 the structure of the podobranchiae shows the distinctness of this form 

 from Astacopsis. 



The Branchial formula of Chaeraps (?). 



6 + ep r+6 +5 + 4 =21 + ep r. 



5. The Branchice of Engaeus. 



The number, the general disposition, and the structure of the 

 branchiae are the same as in Astacojisis and Chceraps. 



The podobranchiae have no vestige of a lamina. The first podo- 

 branchia has a small ala, much as in Chceraps ; but only a few of the 

 branchial filaments have terminal hooks. 



The arthrobranchiae are very small, those of the posterior series 

 being the smaller ; and the three anterior pleurobranchia? are much 

 smaller than the hindermost. 



In all the branchiae, and especially in the arthrobranchiae and pleuro- 

 branchiae, the terminal filament is exceptionally long and thick. 



