PHYLLOPODA. 



349 



appendages ; and the jaws have been recognised in a few 

 instances, either detached or in connection with the body of 

 their former possessor. Some species of the genus reached 

 a great size, 0. Ludensis growing to a length of two feet. It 

 has also been shown that the caudal segments of some speci- 

 mens bear gill-like plates freely depending from the under 

 surface ; and these organs, which have been compared with 

 the supplementary abdominal gill - feet in 

 Ncbalia, by Dr Henry Woodward, were prob- 

 ably present in all the species of this genus. 

 Dithyrocaris (fig. 204), of the Carboniferous, 

 is allied to the preceding in general form, 

 but it has only one free body -ring. The 

 telson consists of three spines, and the semi- 

 oval valves are marked by conspicuous longi- 

 tudinal ridges. 



Another remarkable group of Phyllopods, 

 of which all the known forms except the 

 Triassic Aspidocaris are Palaeozoic, may be 

 considered to be typified by the Silurian 

 genus Peltocaris. In this genus the carapace 

 (fig. 203, d) is approximately circular, and 

 consists of two valves of a semicircular 

 form, which are united along the back by a 

 hinge. The valves, however, are so notched in front as to 

 leave a space which is completed by the addition of a third 

 parabolic valve or " rostrum." The entire carapace is con- 

 centrically striated, but the body-rings are unknown. The 

 Discinocaris of the Silurian (fig. 203, c) is very like the 

 preceding, but the two lateral valves are anchylosed, and the 

 rostrum is triangular. One species of this genus attains a 

 diameter of seven inches across the carapace. Aptychopsis, 

 also Silurian, has the straight "medial suture" and triangular 

 rostrum of Discinocaris, but the valves are united by a dorsal 

 suture, as in Peltocaris. Pterocaris, from the same formation, 

 resembles the genus just mentioned, but the valves are only 

 united by suture for a short space anteriorly, while they 

 diverge behind, so as to form a wide posterior notch. Lastly, 

 in the Silurian Cryptocaris the carapace is semicircular, and 



Fig. 204,Ditliyro- 

 caris Scouleri, view- 

 ed from above, re- 

 duced slightly in 

 size. Carboniferous 

 Limestone. (After 

 M'Coy.) 



straight median 



