PHYLLOCARIDA. 



515 



Lastly, in the Carboniferous and Devonian genus Dithyrocaris (fig. 

 367), there is a broad carapace composed of two semi-oval valves 

 ridged along the back in the middle line and laterally, and there is 

 only one free body-ring. The rostrum is unknown, but there is a 

 powerful trifurcate telson. 



Another remarkable series of the Phyllocarida comprises forms 

 with a thin, concentrically striated carapace, which may be simple, or 

 may be divided into two halves and sutured along the back, and which 

 is commonly notched in front for the reception of a triangular mov- 

 able " rostrum." A number of curious, and in some cases prob- 

 lematical, fossils, the oldest of which are found in the Ordovician, 

 while the last appear in the Trias, have been included in this 

 group, and only a few of the more important can be noticed here. 

 A typical genus of this group is the Peltocaris of the Ordovician 

 rocks (fig. 365, d), in which the carapace is approximately circular, 



Fig. 366. — Carapace of Aris- 

 tozoe memoranda, from the 

 Silurian rocks of Bohemia, of 

 the natural size, viewed side- 

 ways, with the dorsal margin to 

 the left. (After Barrande.) 



Fig. 367. — Dithyrocaris 

 Sco it le ri, viewed f r o m 

 above, reduced slightly in 

 size. Carboniferous Lime- 

 stone. (After M'Coy.) 



and consists of two valves of a semicircular form, which are united 

 along the back by a straight dorsal suture, and which are deeply 

 notched in front for the reception of a movable parabolic plate or 

 " rostrum." The Ordovician and Silurian genus Aptychofisis is very 

 similar in general characters to Peltocaris, but the rostrum is trian- 

 gular in form. In the Silurian genus Pterocaris the same general 

 type of structure is retained, but the valves are only united for a 

 short space anteriorly, while they diverge behind, so as to form a 

 wide posterior notch. On the other hand, in the remarkable Silurian 

 genus Discinocaris (fig. 365, c), the concentrically striated carapace is 

 circular in form, and shows no median line of suture on its dorsal 

 surface, while there is a deep triangular notch in front for the recep- 

 tion of the rostrum. One species of this genus attains a diameter 



