S. Woodward — Notes on PalcBOzoic Crustacea. 



435 



ties, and slightly overlap tlie following segment. The four following 

 segments have the borders of their epimeral pieces rounded, and 

 gradually decrease in breadth downwards fi-om 9 lines to 7, and 

 increase in depth from -| line to 1 line. 



A section of one of the segments would present an outline lite 

 that of Pliacops among the Trilobites, namely, a triple corrugation. 



The 6th thoracic segment is more strongly arched than the preced- 

 ing 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 ^^^^^^^ ^^^j^j-— ~ ^-^^ , 

 three segments, each 2 lines in 

 breadth, and \^ line in depth. 



The first has no epimera, and ^ j^^^^^^g^^^-t^— -t ^^zzj:^; - 

 appears to move freely at its ^^^^ ^^^^^^^^^ a) 



articulation with the last thoracic ^^^^^^ ^^^^^^P ^ ' 



segment. The second and third ^^^^^^^^^^B" 'ti * 



segments have small epimeral ^^^^^^^^^ 6 ) 



pieces, which are bilobed, with ^^^'^^^^g^' •' 



the posterior lobes more pointed. ^ ^^^f ' 



A line of small tubercles runs ^ ^^S 8 / ab 



down the centre of these three ^^^s^r" 



joints, which are somewhat raised cl^.' i "' S ° 



at their articular borders. 



The telson is 1 2 lines in length 

 and Ij 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 „ . .,.,„„ 



, T . T "^ , ° Fig. 1. — Memiasins limuloides, H. \\ood- 



as abdominal, we must presume ward,L. Ludlow, Leintwardlne. 



that each of these latter is a ^, the liead;f a, the sis thoracic segments ; 



"^ ^'' ^ "^ "" ab, the three ahdonunal somites ; t, the telson. 



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 paper on 

 the structure of the Xipliosura} 



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

 normal number of six to three segments, seems to indicate a form by 

 which, with the help of others, we may bridge over the interval that 

 has heretofore existed between these two groups, the Earyptevida and 

 the X.ipliosura. 



Although Hemiaspis is the only genus met with in Britain having 

 this remarkable form, we know of three Russian genera which pre- 

 sent almost identical peculiarities of structure. Dr. J. Nieszkowski 

 has described two forms from the Upper Silurian of the Island of 



^ See Quart. Journ. Geol. Soc, 1867, vol. xxiii. p. 28, plates i. and ii. 



