53- 



CRUSTACEA. 



the same age, is chiefly separated from the former by its much more 

 convex glabella. Lastly, Bathywus (Upper Cambrian and Ordovi- 

 cian) and Bathyurellus (Ordovician) form in some respects inter- 

 mediate types between the Conocephalidce and the Asaphidce ; and 

 the Olenoid genus Triarthrus survives to near the middle of the 

 Ordovician period. 



Family 5. BoHEMiLLiDiE. — This family contains only the Ordo- 

 vician genus Bohemilla, characterised by its greatly developed head ; 

 the glabella being of large proportionate size, with four pairs of 

 lateral grooves, the hinder ones of which pass completely from side 

 to side, so as to divide this region of the head into regular rings or 



Fig. 388. — Angelina Sedgwickii, in its natural condition, and distorted by cleavage. 

 Upper Cambrian. (After Salter.) 



segments, closely resembling those of the thoracic axis. The eyes 

 are large and reticulated ; the course of the facial sutures is un- 

 known ; the genal angles are prolonged into long spines, directed 

 transversely rather than backwards ; and the thorax and pygidium 

 are not clearly marked off from one another, the former consisting 

 apparently of five segments and the latter of two. 



Family 6. Calymenid^. — In this family the crust is often tuber- 

 culated or granulated ; the head is well developed ; the glabella is 

 widest behind ; and the facial sutures are discontinuous, and 

 terminate at the posterior angles of the cephalic shield. There 

 are thirteen segments in the thorax, with grooved pleurse ; the tail 

 is of moderate size \ and the condition of the eyes is variable. In 



