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



angle of the antennal regions, and ventrally between the upper surface of the second 

 antenna and the under surface of the antennal region (PL XIII. figs, a be). 



The dorsolateral margin of the carapace forms an angular ridge, and is divided more 

 or less distinctly into three portions in separate species. These divisions correspond to 

 those seen in the Scyllaridae, as exemplified most clearly in Arctus orientahs (PL IX. 

 fig. 4) ; the anterior represents the line of the cervical fossa, while the posterior may be 

 called the siagnotic fossa, as the space between the two is occupied by the siagnos, or 

 mandible, and may well be termed the siagnotic region. This latter sulcus, although 

 strongly defined in some species both in the Scyllaridae and in the Eryonidse, is in others 

 less appreciable, and disappears in Ibaccus, and is determinable only by an analytic 

 examination in the Astacidse. 



These several divisions of the carapace in the animals belonging to the Willemcesian 

 group are defined by a series of sharply-pointed teeth that differ in number and vary in 

 size and proportions in different species, and may safely be used as a convenient and 

 fairly reliable sign of specific structural difference. 



In the dorsal median line there is a longitudinal carina more or less important, 

 culminating in strong teeth on each somite of the pleon, and taking the unusual 

 direction of pointing forwards. It commences on the frontal margin of the carapace, and 

 terminates in the anterior extremity of the telson. On the carapace it is scarcely more 

 prominent than a small ridge that in some species is only granulated. It is generally 

 armed with teeth on the posterior margin of the cervical sulcus, the gastric region, and 

 the frontal margins, where one or more generally stand in an oblique position behind the 

 edge, which is not produced to a point or rostrum as in Astacus, &c. These teeth 

 correspond with those in the same position, and further resemble them in being some- 

 times double in Arctus and allied genera, but are more robust. One peculiar feature 

 in the external structure of these animals is the manner in which the first somite of the 

 pleon interlocks with the carapace. In Polyclieles and Pentacheles a tubercular process 

 (peltecleis) of the posterior margin of the carapace projects obliquely backwards 

 (PL XVI. fig. 4), and overlaps the anterior margin of the first somite of the pleon, 

 while the lateral extremity of the first somite, or that portion which is laterally external 

 to the tubercular process of the carapace, is directed forwards, and overlaps the pos- 

 terior marginal angle of the carapace. 



In the genus Willemcesia the interlocking is different. There the peltecleis, instead 

 of overlapping the anterior margin of the first somite of the pleon, projects into a 

 hollow socket beneath the anterior margin of the same somite, and acts as a bolt and 

 joint (PL XX. fig. 1). In the Scyllaridse there is no peltecleis, but the anterior lateral 

 margin of the first somite of the pleon overlaps the posterior lateral angles of the 

 carapace, and so securely holds it down. 



In the Palinuridae a lateral process of the first somite of the pleon overlaps the 



