776 PROF. T. H. HUXLEY ON THE CLASSIFICATION [JlUie 4, 



as those which beset the stems of the podobranchiae, have hooked 

 apices. 



In the Potamobiidae the podobranchiae of the second, third, 

 fourth, fifth, and sixth thoracic appendages are always provided with 

 a large plaited lamina. The pedobranchia of the first maxillipede is 

 converted into an epipodite, entirely devoid of branchial filaments. 



The first abdominal somite invariably bears appendages in the male, 

 and usually in both sexes. In the male these appendages are styli- 

 form, and "those of the second somite are always peculiarly modified. 



The appendages of the four following somites are relatively small. 



The telson is usually completely divided by a transverse suture. 



None of the branchial filaments are terminated by hooks ; nor are 

 any of the coxopoditic setae or the longer setae of the podobranchiae 

 hooked, thougli hooktd tubercles occur on the stem and on the lamina 

 of the latter. 



The coxopoditic setae are always long and tortuous. 



It is worthy of notice that the Parastacidte agree with the 

 Palinuridse and Scyllaridae in the abortion of the appendages 

 of the first abdominal somite, and in possessing hooked setae, while 

 in the Potamobiidae, as in the Lobsters, the setae are not hooked, 

 and, as in almost all the Podophthalmia, except the Palinuridae and 

 Scyllaridae 1 , the appendages of the first abdominal somite are present, 

 and are specially modified in the males. 



Of the six genera of the Para*tacidae, Astacoides is widely sepa- 

 rated from all the rest by the reduction of the number of its func- 

 tional branchiae to twelve, while all the other genera at present known 

 have, at fewest, twenty, and usually twenty-one, brancbiae. 



Astacopsis, Chceraps, Etigaus, and Pnrastacus have, in common, 

 a long epistoma, the surface of which is flattened, the basal joint 

 of the antennae fixed by the overlapping edge of the cephaloste- 

 gite, the posterior thoracic sterna narrow, the coxopodites of the 

 hinder thoracic limbs large and approximated in the middle line. 

 The rostrum and the antennary squame are short. Engmus is dis- 

 tinguished among these, not only by the narrowness of the first ab- 

 dominal somite, but by the form of the anterior part of the head, 

 with its short and deflexed rostrum, and very small antennary squame. 



In Chcrraps the podobranchiae are alate, in Astacopsis they are 

 not. Parastacus somewhat resembles Chceraps in its alate podo- 

 branchiae, but differs from all the rest by possessing long and tortu- 

 ous coxopoditic setae. 



I have nothing to add to the distinctive characters of the two genera 

 of the Potamobiidae, Astacus and Cambarus, already given by 

 Erichson, Hagen, and others. 



All the branchiae of the Crayfishes consist of a stem beset with nu- 

 merous cylindrical filaments. In fact, they are typical examples of 

 what are termed by Milne-Edwards " branchies en brosse," and may 

 be called " trichobranchiae," in contradistinction to the lamellar 



1 In Gebia, Calliaxis, and Porcellana, the first abdominal appendages are 

 rudimentary or abortive in the male sex. 



