T H A L A S S I N I D E A. 507 



as in Gebia and Callianassa. The parts in the Astaci and Thalassi- 

 nidea have, consequently, like relations. 



In the Macroura, therefore, it is probable that the anterior part of 

 the carapax corresponds generally to the second antennary normal 

 segment, and the posterior and lateral to the mandibular segment, the 

 lateral portions being epimeral to the segment last-mentioned. The 

 former corresponds to the main body of the Brachyural carapax, the 

 latter, to only its ventral pieces (epimerals of Edwards) separated from 

 the rest by a longitudinal suture. 



But before the difficulties of this subject are wholly cleared up, we 

 must consider more particularly the actual relations between the longi- 

 tudinal suture in the Brachyura generally, and those in J3glea, Pagu- 

 rus, and the Thalassinidea. This suture of the Brachyura exists dis- 

 tinctly in Galathea, and is the only suture ; and in this genus of Ano- 

 moura, therefore, the carapax must have the same normal relations as 

 in the Brachyura. In yEglea, a genus near Galathea, we find the Bra- 

 chyural and Macroural sutures strangely combined ; there is a lateral 

 (Z), the analogue of the Brachyural, and besides this, another longitu- 

 dinal (p), nearer the middle of the back, and the latter is the more 

 distinct. The same is shown also in Pagurus. Such transition pecu- 

 liarities in these transition forms are of great interest, and at first 

 thought, are perplexing to the mind. The next grade (represented 

 among the Thalassinidea and very many Paguridae) presents but one 

 of these sutures, the other being obsolete ; and from the much greater- 

 distinctness of the inner, in Pagurus and JEglea, we infer naturally, 

 that it is the outer or true Brachyural suture that has become obso- 

 lete. This would also be inferred from the resemblance in the medial 

 area between Pagurus and Thalassina ; the greater width of this area 

 in Gebia does not seem to vitiate the conclusion ; for we find both the 

 broad and narrow form in the Astaci. 



These facts might be thought to prove, that the suture in the 

 Brachyura, as Milne Edwards has argued, is only an epimeral suture, 

 inasmuch as the longitudinal suture corresponding, is so subordinate 

 in the Paguri and related forms.* But if we remember that these are 

 only transition forms, and this is but the dying out of the Brachyural 

 structure, as the species pass to the Macroural type, we shall not 

 thus conclude. The suture I (corresponding to the Brachyural), is 



* With regard to the Macroura, M. Edwards supposes the posterior region epimeral to 

 thu anterior. 



