1865.] H. WOODWARD SILURIAW CRUSTACEA. 491 



the carapace terminate in a broad triangular point directed back- 

 wards. Two lesser spines arm 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 (see 

 Plate XIV. fig. 76). 



The first segment is the largest, being 1 line in depth and 7| lines 

 in breadth, including the epimera, which are pointed at their extre- 

 mities and slightly overlap the following segment. Three minute 

 tubercles ornament the median portion of each segment. The four 

 following segments have the borders of their epimeral pieces rounded, 

 and gradually decrease in breadth downwards from 9 Hnes to 7, and 

 increase in depth from ^ line to 1 line. 



A section of one of the segments woidd present an outline like 

 that of Phacops among the Trilobites, namely a triple corrngation 

 (Plate XIV. fig. 76). 



The sixth thoracic segment is more strongly arched than the pre- 

 ceding 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| line in depth. The first has no epimera, and appears 

 to move freely at its articulation with the sixth thoracic segment. 

 The second and third segments have small epimeral pieces, which are 

 bilobed with the posterior lobe more pointed. A line of small tuber- 

 cles runs down the centre of these three joints, which are somewhat 

 raised at their articulations. 



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, as in the Xi^^hoswra. 



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 Euri/pterida'^ and the 

 Xipliosu7xi. 



There are several peculiarities about Hemiaspns which seem to offer 

 analogies with the Trilobites, but we know so little of the structure 

 of that very isolated group that we cannot venture to speculate on its 

 affinity to this order. 



Note. — Mr. Salter is acquainted with several species oi Hemiaspis, 

 which have been marked Avith MS. names by him in the Jermyn 

 Street Museum. They are, however, extremdy fragmentary. The 

 species are as follows : — 



■* Among the Euryj)fenda, perhaps Sfi/Ion urns Powr/ci comes nearest in gene- 

 ral form ; but Hemia^pis will be seen to differ widely even from this genus. 



