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



are anteriorly produced beyond the frontal margin. The anterior and two-thirds of the 

 lateral margins are smooth, whereas the posterior portion is armed with five prominent 

 teeth. The median dorsal line is longitudinally armed with three or four prominent 

 teeth, one being strongly marked on the frontal margin, a second imperfectly present 

 over the gastric region, a third and fourth over the pyloric and cardiac regions, and 

 evidence exists of a double row of bead-like tubercles longitudinally traversing the median 

 line from the posterior to probably the anterior margin. 



The inner line of the branchial region is posteriorly defined by a low ridge fur- 

 nished with three or more small points or tubercles. From the gastric region to the 

 lateral margins of the carapace, a strong ridge traverses the line of the cervical fossa in 

 recent Crustacea, a circumstance that I believe is due to compression during fossilisation ; 

 the weaker parts yielding while the more rigid and stronger resist. Thus the fossa which 

 is due to a reflexion or folding of the dermal tissue resists more decidedly the superin- 

 cumbent pressure and remains rigid, while the surrounding structure yields. The 

 cervical fossa, or as it may be called in this specimen, ridge, bifurcates into an anterior 

 and a posterior branch, between the fork of which lies what Stimpson has called the 

 hepatic region. 



The posterior portion or pleon is broad, and symmetrically tuberculated ; each somite 

 •generally carries, or is supposed to carry one large tooth or tubercle on the posterior 

 margin in the median line, a similar but larger one near the lateral margin, centrally 

 situated above the coxal plate, and another smaller in dimensions between this and the 

 central, is situated on the posterior margin. 



The animal appears to have no ophthalmopoda, although a semicircular notch in the 

 frontal margin of the carapace seems to represent the orbit of the missing organ. This 

 absence may, and I believe does arise from the soft and perishable nature of the eye 

 when compared with the surrounding tissue, during the period of fossilisation, or it may 

 be from the organ being hid, or reduced to a minimum condition as observed in the 

 Wdlemcesia, or from its entire absence, as in Eryoneicus, but the presence of an orbital 

 cavity determines that this ancient form has departed from a species in which the eye was 

 an important feature. 



Matrazine, for the drawing of which the author is not responsible, a note and additions to the text were added by the 

 editor after the paper had been seen by the author. A comparison of the figure in the magazine with that on pi. xxv. 

 in the Quarterly Journal of the Geological Society, both of which were drawn under the superintendence of the editor 

 of the Geological Magazine, will show that the form of the large chela and the ornamentation at the branchial margin 

 of the carapace cannot belong to the same species. More than twenty species of Eryon have been described, but 

 these are so various in their external form that it is difficult to imagine that they are not structurally, more than 

 specifically distinct. As an example, Dr. Woodward, in a note to his Memoirs on the Species of the Genus Eryon 

 (Uesm.) from the Lias and Oolite of England and Bavaria (loc. cit., p. 494), tells us that the direresis " is absent in the 

 outer caudal lamella? of the Solonhofen species — a most important distinction : they differ also widely in form," and he 

 describes all the English species as possessing it, " as in other Astacidne." The diaeresis is absent from the following 

 recent families: Eryonida\ Palinuridas and Scyllaridse, all of which belong to the Astacidea. 



