114 THE VOYAGE OF H.M.S. CHALLENGER. 



formis, and that of Astacus was given by Kichter. It has also been figured by 

 Desmarest under the name of Eryon cuvieri, in his Crustaces Fossiles, and the figure 

 was afterwards reproduced in his Considerations des Crustaces. Since that Count 

 Munster, as well as Mr. Woodward, has described and published the figures of several 

 species, in a monograph on the Merostomata. 1 



The general resemblance in form of the species belonging to the genus Eryon to 

 Polycheles crucifera is very close, both in the dorsal aspect of the carapace and in the 

 character and arrangement of the pereiopoda. The pleon also with its terminal rhipidura 

 bears a closely corresponding relationship. 



An analytical examination of the several parts of the recent form demonstrates a 

 variation in structure of a very decided and distinguishing character, when compared 

 with the Solenhofen specimens. 



Except in the recent forms related to Polycheles, the eyes are so impoverished as 

 to be overlooked except on close examination, and then they are observed to pass beneath 

 the outer or frontal angle of the carapace as in the annexed woodcut (fig. 28). 



If we turn to Eryon, the appendage that is supposed to be the foot-stalk of the eye 



is situated at the extremity of a prominence projecting from the frontal margin of the 



.;._ carapace external to the antennae. This is so con- 



:"•. j \ stant among the specimens that, abnormal as it may 



appear, we must accept it as being a feature in the 

 structure of at least one genus of the group. The 

 outer or antennal angle is, therefore, not produced 

 anteriorly as in Polycheles, or externally as in Ibaccus 

 Yia.m.-Pentacheks'gZuis. Ophttaimopod and Arctus, but recedes posteriorly from the orbit. 



and frontal margin of carapace. The first pair of antennas in Eryon has three cylin- 



drical joints terminating in two flagella, not so short as in the Scyllaridae, but 

 very much shorter than in Polycheles. In this latter form the first joint of the 

 peduncle of this antenna is developed on the inner side into a broad thin plate 

 that is forced upwards by lateral pressure, while in Eryon the joint is simply sub- 

 cylindrical. 



The second pair of antennas is robust, and, according to Desmarest, with a large scale 

 at the base, which is not shown in his figure, but is understood by naturalists to mean the 

 scaphocerite. 



An examination of the specimens in the British Museum, which were courteously 

 placed at my disposal by Dr. Woodward, F.E.S., has convinced me that " une ecaille assez 

 large, ovo'ide et forternent echancrde du cote interne " does not always mean the scapho- 

 cerite, but sometimes refers to a aquamiform extension in the breadth of the penultimate 

 joint of the peduncle. This is well seen in Eryon lotus, Munster (No. 44818 in the 



1 Travs. Palwont. Soc, 1866 ; and Quart. Joum. Geol. Soc, vol. x.\ii. p. 494, 1866. 



