5 14 CRUSTACEA. 



valves, contracted in front and with an abrupt posterior truncation. 

 The carapace-valves are united along the back by a median line of 

 attachment, and are commonly marked with fine linear striae, while 

 a lanceolate rostrum is developed in front. There were fourteen 

 or more body-rings, of which the last five or six were free, some or 

 all of these supporting lamellar appendages which seem to have 

 been branchial in function. At the hinder end of the body is a 

 powerful pointed telson, with two smaller lateral spines. The 

 thoracic limbs are unknown, but toothed jaws have been in some 

 instances recognised, either detached or in connection with the 

 body of their former possessor. The oldest species of Cei'atiocaris 

 appear in the Ordovician rocks, but the genus attained its maximum 

 in the Silurian period, and the last known forms occur in the Lower 

 Carboniferous deposits. Some of the species attained to a great 

 size, C. Ludensis growing to a length of two feet. 



In the Cambrian rocks is found the allied genus Hymenocaris 

 (fig. 365, b\ in which the large sub-triangular carapace is not bi- 

 valved but is simply folded. There are generally nine free abdom- 

 inal segments, the hinder termination of the body being adorned 

 with three pairs of unequal spines. The Cambrian rocks of North 

 America have yielded the remains of the related genus Protocaris, 

 which differs from Hymenocaris principally in the possession of 

 thirty narrow abdominal rings, and in the fact that there is a large 

 telson ending in two terminal spines. Of the same general type 

 as the preceding is the Ordovician genus Caryocaris, which agrees 

 with Ceratiocaris in the fact that the carapace is bivalved, instead of 

 being simply folded. The carapace is pod-like in form, and trun- 

 cated behind, and the abdomen terminates in three spines. The 

 researches of Novak have, further, shown that the singular Crus- 

 taceans described by Barrande from the Silurian rocks of Bohemia 

 under the names of Aristozoe, Callozo'e and Orozoe, and originally 

 regarded by this observer as gigantic Ostracodes, are really referable 

 to the Ceratiocaridce. The carapace of Aristozoe (fig. 366) reaches 

 three inches in length, and is composed of two valves united along 

 a straight dorsal margin, its general shape being oval, with a pointed 

 anterior and rounded posterior margin, and its front portion carrying 

 prominent rounded tubercles. Novak has shown that the fossil 

 described under the name of Bactropus is really a portion of the 

 abdomen of Aristozoe, and that the so-called Ceratiocaris debilis is 

 the telson of the same genus. 



Of the later forms of the Ceratiocaridce, the Devonian rocks of 

 North America have yielded the remains of the curious genus 

 Echinocaris, in which there is an ovoid folded carapace, adorned 

 anteriorly with tubercles, and apparently without a rostrum. There 

 are seven free abdominal segments, and a strong trifurcate telson. 



