MEROSTOMATA. 



549 



(fig. 412); and the abdomen consists of three or more rings, and 

 is terminated by a spine-like telson. The appendages are as yet 

 unknown. 



The genus Hemiaspis (fig. 412, a) comprises some singular Silurian 

 Crustaceans, in which there is a semicircular head-shield with a central 

 glabella, and indications of facial sutures and eyes. The thorax is tri- 

 lobed, and consists of six free segments. There are three free segments 

 in the abdomen, and these are followed by a long spiniform telson. 

 The genus Pseudoniscus, also from the Silurian rocks, resembles Hemi- 

 aspis in many points, and particularly in the possession of free thoracic 

 segments, but there is no distinct line of demarcation between the thorax 

 and the abdomen (fig. 412, B). Another Silurian genus, also closely re- 



Fig. 412.— a, He7niaspis limuloides (after H. Woodward) ; b, Pseudoniscus aculeatus (after 

 Nieszkowski) ; c, Bunodes (Exapinurus) Schrenkii (after Nieszkowski). All from tbe Silurian. 



lated to the preceding, is Bimodes (Exafti?inrus), in which the thorax 

 likewise consists of free rings, but the last two are fused with one another. 

 There are three movable abdominal rings and a spine-like telson (fig. 

 412, c), and the animal possessed, like so many of the Trilobites, the 

 power of rolling itself into a ball. 



In the remarkable genus described by Dr Henry Woodward from the 

 Silurian rocks of Britain under the name of Neolimulus (fig. 413), the 

 head-shield has a general resemblance to that of the recent Limulus, and 

 there are traces of a divisional line crossing the head and apparently cor- 

 responding with the "facial suture" of the Trilobites. Compound eyes 

 and ocelli seem to be present, and there are six free thoracic segments, 



