326 Shield-bearing Crustacea. 



Limuli, apparently differing but little as regards the 

 carapace from the recent species of Molucca and America (see 

 plate, Figs. 1 and 2), occur as early as the deposition of the 

 Solenhofen limestone in Bavaria (see plate, Fig. 3); and in the 

 Coal-measures of England and Ireland, several species of 

 Bellinuri occur (see plate, Figs. 4, 5), in which the cephalic 

 shield is composed of the cephalo-thorax ; and the segments of 

 the abdomen, if not anchylosed in all,* are so in most. 



But in Hemiaspis Umuloides we have the cephalic, thoracic, 

 and abdominal divisions still remaining distinct, and apparently 

 capable of separate flexure. This important character at once 

 separates it from Limulus and Bellinurus. 



It will also be observed that Hemiaspis is, in general ap- 

 pearance, strongly severed from the other species of Eury- 

 pterida, as well as from the Xiphosura in structure. The three 

 divisions into head, thorax, and abdomen are more strongly 

 marked. The abdomen is reduced to very slender proportions 

 (less than one-third the breadth of the thoracic plates), and 

 the telson, or tail-spine, is nearly one-third the length of the 

 entire animal. 



The carapace in general outline resembles Limulus, but is 

 more dilated laterally. 



The glabella, or central portion of the shield, is ornamented 

 with tubercles. 



The thorax is composed of six strongly trilobed segments, 

 the epimera (lateral portions) being equal in breadth to the 

 central portion of each segment. 



These segments present a striking analogy to the trunk- 

 segments of a Trilobite. 



The abdomen consists of only three segments, the second 

 and third having small bilobed epimeral pieces. 



If we regard the first six body-rings 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. (See plate of Slimonia, 

 vol. iv., p. 229.) 



On the other hand, the presence of these three segments 

 precludes our considering the head to be the cephalo-thorax, 

 and the succeeding- segments the abdomen, as in the 

 Xiphosura. 



The smallness of the abdomen, and its reduction from the 

 assumed normal number of six segments to three, seems to 

 indicate a form by which, with the help of others, wo may 

 bridge over the interval that has hitherto existed between these 

 two groups, the Eurypterida and the Xiphosura. 



* In Bellinurus regina (Fig. 5) the abdominal segments are considered by Mr. 

 Baily to be moveable. 



