176 BRITISH FOSSIL CRUSTACEA. 



as raised tubercles on the posterior border of the head-shield, three lines apart, gradually 

 converge and unite, so as to form an arch, the summit of which nearly touches the 

 front border of the glabella. 



Nine ray-like corrugations descend from the glabella towards the margin of the 

 shield, and the whole surface of the carapace is very minutely granulated. The lateral 

 margins of the shield are ornamented with minute spines, and the rounded posterior angles 

 of the carapace terminate in a broad triangular point directed backwards. A fringe of 

 lesser spines arms the lateral border of the glabella. 



The thorax is composed of six strongly trilobed plates ; the epimera being equal in 

 breadth to the central portion of each segment. 



The first segment is the largest, being 1 line in depth and 1\ in breadth, including 

 the epimera, which are pointed at their extremities and slightly overlap the following 

 segment. The four following segments have the borders of their epimeral pieces 

 rounded, and gradually decrease in breadth downwards from 9 lines to 7, and increase in 

 depth from -| line to 1 line. 



A section of one of the segments would present an outline like that of P/iacops 

 among the Trilobites, namely, a triple corrugation. 



The sixth thoracic segment is more strongly arched than the preceding ones, and the 

 lateral borders are divided into two rounded lobes on each side : breadth 5 lines, depth 

 1 line. 



The abdomen consists of only three segments each, 2 lines in breadth and 1^ lines in 

 depth. The first has no epimera, and appears to move freely at its articulation with the 

 six thoracic segment. The second and third segments have small epimeral pieces, which 

 are bilobed, with the posterior lobe more pointed. A line of small tubercles runs down 

 the centre of these three joints, which are somewhat raised at their articular borders. 



The telson is 12 lines in length and 1|- line in breadth where it articulates with the 

 abdomen. It tapers gradually to a fine point. 



If we regard the first six body-rings from the head as thoracic, and the remaining three 

 segments as abdominal, we must presume that each of these latter is a double segment, 

 as compared with the segments of the Eurypterida proper. 



On the other hand, the presence of these three segments precludes our considering 

 the head to be the cephalothorax and the succeeding segments the abdomen, a view 

 controverted by me in my examination of the structure of the Xiphosura) 



The smallness of the abdomen, and its reduction from the assumed normal number of 

 six to three, seems to indicate a form by which, with the help of others, we may bridge 

 over the interval that has hitherto existed between these two groups, the Eurypterida and 

 the Xiphosura. 



Although Hemiaspis is the only genus met with in Britain having this remarkable 

 form, we know of three Russian genera which present almost identical peculiarities of 



1 See paper by the author, ' Quart. Journ. Geol. Soc.,' 1867, vol. xxiii, p, 28, pi. i and ii. 



