434 H. Woodivard — Notes on Palceozoic Crustacea. 



But in the specimen under consideration we have the cephalic, 

 thoracic and abdominal divisions still remaining distinct, and ap- 

 parently capable of separate flexure. This important character at 

 once separates it from Limulus, Bellinurus, and Frestwichia. I have 

 on this account^ not used the MS. name of Limuloides as a 

 generic appellation, but have proposed the name Hemiaspis (from 

 •^fiiovi, half, and a(nr\<i, a sliield), reserving the MS. name Limu- 

 loides for the specific title of the most perfect species of the genus. 

 (See Plate X., Figs. 1, 2.) 



But it will be observed that Hemiaspis is also, in general appearance, 

 strongly severed from the other species of Eurypterida, 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 pro- 

 portions, less than one-third the length of the animal (the entire 

 specimen measuring 2^ inches in length by one inch in width) . 



The carapace in general outline resembles Limulus, but is more 

 dilated laterally. There is a small stellate ornamentation in the 

 centre of each cheek, having five to six rays, and measuring about a 

 line in extent ; but whether this represents the position of the eyes 

 I am quite unable to say. It is unlike the eye of any other member 

 of the group, which causes me to doubt its relation to that organ. 

 It seems more probable that the eyes were placed along the lateral 

 margin of the glabella, not upon the centre of the cheek. 



There is a faint indication on one side of Fig. 1 and on Fig. 2 of 

 a facial suture to the head-shield (as in the Trilobites), with a small 

 aperture upon its border, which may possibly indicate the true posi- 

 tion of the eye, but it is by no means clearly defined. 



The surface of the glabella when perfect (as in Plate X. Fig. 2) 

 appears to have been almost smooth,^ save that it is traversed by two 

 ridges which, commencing 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 head- shield is armed, on each side, near 

 the rounded posterior angles, with two principal spines directed 

 backwards, whilst a fringe of lesser ones ornaments each lateral 

 genal border. 



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 7^ in 

 breadth, including the epimera, which are pointed at their extremi- 



^ With the concurrence of Mr. Salter given at the time. 



* In the original description of the glabella of Hemiaspis limuloides (see Quart. 

 Journ. Geol See, 1865, vol. xxi. p. 490) I have described the glabella //wwrtrf(;<«c/i««? 

 portion, " as ornamented with a semicircle of nine tubercles, and a tenth immediately 

 ■within the circle upon the elevated front, and two small tubercles at the posterior 

 margin." The acquisition of the second specimen (Plate X. Fig. 2) proves this 

 fragment to belong to another species, not to H. limuloides, as formerly supposed. 



