510 MR MALCOLM LAURIE ON THE 



I. Slimonia. 



It has seemed to me advisable to commence with this form, because I have been able, 

 owing to its greater robustness and large size, to make out its anatomy in greater detail 

 than that of the other members of this group. The principal locality for this genus is 

 Lesmahagow, where it occurs at times in considerable abundance. A metastoma, figured 

 by Salter,* from the Ludlow rocks of Leintwardine, as part of Eurypterus punctatus, 

 is much like that of Slimonia in form, and Henderson t refers some fragments from the 

 Pentland Hills to this genus, but in neither locality have good specimens been found. 



Though the general anatomy is well enough known, a short recapitulation of the main 

 facts may not be out of place here. Viewed from the dorsal aspect, the body is seen to 

 consist of a large carapace, followed by twelve free segments, and terminated by a pointed 

 telson. The carapace is sub-rectangular in this form, with a curved anterior and straight 

 posterior margin, and bears two pairs of eyes. The lateral eyes, placed at the front 

 corners of the carapace, are large, ovoid, and indistinctly facetted. The marginal position 

 is common to this genus, and Pterygotus ; Eurypterus, Stylonurus, &c, having the lateral 

 eyes on the dorsal surface of the carapace. The central eyes, or ocelli, which are very small 

 and not facetted, are placed close together near the centre of the carapace. The anterior 

 margin of the carapace is bent over on to the under surface, and the same was probably 

 the case with the lateral margins. 



The first seven free segments of the body are represented by more or less band-like 

 tergites, united by a soft membrane with the corresponding sternites. The last five 

 segments, however, have no 4 distinct tergite, but are cylindrical sclerites, considerably 

 longer and narrower than those of the anterior segments. The body terminates in a telson. 

 which is moderately broad at the base, expands slightly about a quarter of the way down, 

 and then contracting rapidly, ends in a long pointed spine. This spine is triangular in 

 section, and has far more the appearance of a weapon of offence or defence than merely an 

 ornamental termination to the body. 



Turning now to the ventral surface (fig. 9), the anatomy becomes less simple. In 

 the region of the carapace we find a number of appendages and the sub-cordate meta- 

 stoma. The metastoma appears to have been attached in the middle line, and to have 

 extended in front and at the sides over the jaw-like bases of the posterior limbs, which 

 thus worked in a more or less closed chamber. This arrangement has a functional 

 parallel in Thelyphonus among recent arachnids, in which the chelicerse and the mouth 

 are shut in behind, and on the ventral side by the fused bases of the large second pair of 

 appendages. The metastoma is always more or less heart-shaped, the anterior margin 

 being deeply notched. The shape varies in different genera and species of Eurypterids, 



* Mem. Geol. Surv., Mon. i. pi. xi. fig. iv. t Tr. Ed. Geol. Soc, iii. 



