REPORT ON THE CRUSTACEA MACRURA. 101 



deep water was taken off Sombrero, one of the eastern group of West Indian Islands, at 

 a depth of 450 fathoms, which, from its general resemblance to the fossil genus Eryon, 

 excited peculiar attention, and subsequently the Challenger obtained from even greater 

 depths and in very distant localities many specimens that very closely resembled one 

 another in general appearance. Since then species have been taken by Professor Agassiz 

 in the West Indian Seas, and in the Mediterranean by Professor A. Milne-Edwards. 



Detailed examination of the specimens from the several localities demonstrated that 

 however closely they may resemble one another in external form, they yet exhibit 

 important variations showing that they are not so intimately allied as appearances might 

 suggest. 



In some the external arrangement of the numerous teeth and spines varies without any 

 modification of the structural form, whereas in others the external characters appear to be 

 fixed and the internal structure undergoes a considerable amount of important variation. 



The chief anatomical feature, and one from which all the other peculiarities of 

 form arise, is its flattened and dorsally depressed character, particularly at the anterior 

 extremity, where the frontal margin is so closely compressed upon the antennae that they 

 are flattened at the base, and implanted almost in the same horizontal line, while the 

 ophthalmopoda are forced between the second antennas and the external lateral angle of 

 the carapace, so that the utility of the organ of vision is reduced to a minimum. In the 

 genus Willemcesia the ophthalmopoda appear to be obsolete, and in Eryoneicus to be 

 entirely absent. The antero-lateral angle of the carapace in this group of Crustacea is re- 

 presented by what Stimpson has called the " spina antennalis " in the more cylindrical and 

 common forms. This angle, which is not appreciable in Astacus and Homarus, and only 

 represented by a short tooth in Palinurus, is produced to a marked degree in Arctus and 

 Ibaccus, where, in the latter especially, it is carried to a very considerable extent out- 

 wards. But in Polycheles and in Arctus, instead of being directed laterally outwards, 

 it is produced in the same manner, forwards and outwards. The orbit being in a similar 

 position, and formed in a similar way, in the Scyllaridse and Eryonidse, the consequence 

 is the degradation of the organs of vision and the reduction of size and alteration of form 

 of the orbit, which in some species of the Eryonidse results in the almost total suppression 

 of the ophthalniopoda. 



Ranging from the anterior or antennal tooth, the line of anatomical depression 

 produces a lateral crest that longitudinally defines the dorsal from the ventral surfaces 

 of the carapace ; it resembles the lateral margin of the carapace in the Brachyura, but is 

 not homologous with it. In the Brachyura the marginal angle corresponds with the 

 external angle of the orbit, or, according to Stimpson's nomenclature, the "angulus 

 orbitse externus;" the antennal angle, or "spina antennalis," being carried under, forms 

 the inferior margin of the orbit, whereas in Polycheles the inferior surface of the orbit is 

 imperfect, the eye appearing dorsally in a cleft between the frontal and the anterior 



