540 



CRUSTACEA. 



Family 13. Acidaspid^. — Like the preceding, this family contains 

 only a single genus — viz., Acidaspis itself. In this characteristically 

 Silurian type (fig. 400, b), the usual form of the head-shield in the 

 Trilobites is somewhat masked, the trilobation being rendered indis- 

 tinct by the presence of an additional and secondary pair of axal 

 furrows, which mark off a central inflated portion of the glabella. 

 The thorax has nine or ten rings, with ridged pleurse, which are 

 terminated by spines ; while the tail is very small, and has its 

 margin fringed with spines (fig. 399). The facial sutures are 

 continuous, or are sometimes wanting ; and the eyes are of small 

 size and smooth. The species of Acidaspis are found in the Silurian 

 and Devonian rocks, and are usually readily recognised by their 

 highly ornamented and spinose crust. 



Family 14. Lichad^. — The principal or only genus in this family 

 is Lichas 1 itself (fig. 400, a), in which the body is broad and oval, 



and the crust is superficially more or less 

 granulated or tuberculated. The head- 

 shield is transversely elongated and 

 very convex, and the glabella as a rule 

 is not very clearly separated from the 

 cheeks. The frontal grooves of the 

 glabella are extended backwards, so 

 as to enclose a central lobe. The 

 facial sutures cut the anterior margin 

 separately, and the eyes are small and 

 smooth. There are nine or ten tho- 

 racic rings, with grooved pleurae ; and 

 the pygidium is larger than the head, 

 and often presents prominent spinose 

 ends to its component rings. The 

 species of Lichas are Ordovician, Si- 

 lurian, and Devonian. 



Family i 5. Bronteid^. — This 

 family includes only the genus Bron- 

 teus, the species of which are found in 

 the Ordovician, Silurian, and Devonian 

 rocks. In this well-marked type (fig. 

 401) the body is broad, and could be rolled up, both the head- 

 shield and the pygidium being of great size. The tail is particu- 

 larly large, and is always more or less fan-shaped, the axis being 

 short and rudimentary, while the " limb " is greatly developed. 

 The head-shield is trilobed, the glabella being dilated in front, the 



1 The genus Lichas has been very fully treated of by Magister Friedrich 

 Schmidt ('Revision der Ostbaltischen Silurischen Trilobiten,' Abth. II., 1885), 

 by whom it has been split up into a number of sub-generic groups. 



Fig. 401. — A, Head-shield of Bron- 

 tens camftanifer, from the Silurian 

 rocks of Bohemia ; b, Tail of the same. 

 (After Barrande.) 



