TRILOBITA—OPISTHOPARIA. 283 



XXXI. Leptoplastus Angelin. 



Elongate-oval^ Cephalon convex, margined by an elevated nar- 

 row border and a groove within it. Eyes placed in the middle of 

 the cheeks, joined to the* glabella by an ocular ridge. Facial 

 sutures converging in front of the eyes. Genal angles set forward 

 and produced into short, straight spines. Glabella subcylindric or 

 conical. Lateral furrows oblique. Thorax of 11 or 12 segments. 

 Pleura straight, grooved, short, pointed at the ends. Pygidium 

 minute ; margin with short spines. Cambric. 

 70. L. spinosus Matthew. (Fig. 1584.) Cambric. 



Hypostoma very similar to that of Ctenopyge acadica but shorter 

 and without the marginal fold in front. Free cheeks wider than 



Fig. 1584. Leptoplastus spinostis : a, middle part of head shield ; b, free cheek; <:, 

 segment of thorax ; d, pygidium ; e, young hypostoma. (After Matthew.) 



long. Pygidium with prominent axis and three short, reflexed 

 spines on each side. 



Upper Cambric (Bretonian) of New Brunswick. 



XXXII. Crepicephalus Owen. 

 Like Ptychoparia but with projecting posterolateral spines on the 

 pygidium ; these are outgrowths from the pygidium as a whole and 

 not terminations of single anchylosed segments. Cambric. 



71. C. augusta Walcott. (Fig. 1585.) Cambric. 

 Surface of the larger specimens papillose. Pygidial spines short. 

 Middle ( ?) Cambric Pioche formation of Nevada. 



72. C. texanus Shumard. (Fig. 1586.) Cambric. 

 Large. Marginal fold of cephalon wide. Pygidium with two 



strong, curved spines. 



Middle Cambric of Alabama, Texas and Wyoming. Middle and 

 Upper Cambric (Weeks and Orr formations) of Utah. 



