508 



CRUSTACEA. 



wholly absent, as stated, in most of the Paguridae, and is but faintly 

 seen in any of them ; while the transverse suture becomes the grand 

 suture of division in the Macroural carapax, being the most promi- 

 nent, and the last to disappear as the species descend in rank. 



From the remarks which have been made, it is clear that there are 

 at least three distinct varieties of structure among the Thalassinidea, 

 one illustrated by Gebia, one by Callianassa, and one by Thalassina. 

 There are other characters which point to these as three important 

 subdivisions of this group. These subdivisions pertain to the section 

 of the Thalassinidea having thoracic branchiae alone. The other 

 division, in which there are abdominal branchial appendages, contains 

 but two genera. The following is, therefore, our classification of the 

 Thalassinidea : 



Legio I. THALASSINIDEA EUBRANCHIATA. 



BRANCHIIS THORACICIS INSTRUCTA TANTUM. 



Fam. I. Gebim. — Maxillipedes externi pediformes. Appendices 

 caudales et aliae abdominales latae. 



Fam. II. Callianassid^:. — Maxillipedes externi operculiformes. Ap- 

 pendices caudales latae. 



Fam. III. Thalassinim. — Maxillipedes externi pediformes. Appen- 

 dices caudales lineares. 



Legio II. THALASSINIDEA ANOMOBRANCHIATA. 



PEDES ABDOMINIS APPENDICIBUS BRANCHIALIBUS INSTRUCTI. 



The known genera of living species of Thalassinidea are as fol- 

 lows : — 



