THE GENUS PRESTWICHIA. 245 



is glad to seize upon any such salient point of structure which may present itself in these 

 fossil forms ; and, though it may appear trivial, we may fairly accept it, under reservation, 

 until better specimens afford a clearer insight into their anatomical details. 



This species and the next following (P. rotundata) were first figured and noticed 

 (though not described) by Mr. Prestwich in his celebrated " Memoir on the Geology of 

 Coalbrook Dale," published in the 'Transactions of the Geological Society ' for 1840. 

 Fragmentary remains have since been obtained from Airdrie, Dudley, and from Mansfield, 

 in the neighbourhood of Nottingham. 



It is at once readily distinguished from the other Coal-measure Limuli by the more 

 outward direction of the genal spines of the head, giving a curious pointed lateral 

 extension to the shield. The hinder border of the cephalon also curves upwards and 

 outwards, so that the cheek-spines seem to spring from the centre of each side of the head, 

 instead of from the postero-lateral border. 



The frontal doublure is strongly marked, as in Bettinurus regince. The details of the 

 eyes and ocelli cannot be well made out, but from the detached head (PI. XXXI, fig. 6 a), 

 one is led to conclude that the glabellal border was marked by a doubly arched front, as in 

 the Bellinuri. The back of the head-shield at its articulation with the axis of the thoracic 

 somites is marked by three prominent spines directed backwards. The thoracico-abdo- 

 minal somites are coalesced, and form a heart-shaped post-cephalic buckler. The 

 border on either hand is marked by six remarkably long slender recurved spines, those of 

 the thoracic somites being as much as seven lines in length. The telson is seven lines 

 long, but it evidently went beyond the circumference of the Clay-ironstone nodule (in 

 which it is embedded) into the shale, and its extremity is therefore lost. The narrow but 

 strongly marked axis of the hinder shield was apparently ornamented by a single row of 

 tubercles down the centre of the somites. Traces of three long slender-jointed limbs are 

 seen projecting from beneath the head-shield in Professor Prestwich's specimen ; this is 

 the only indication of limb- appendages which these Coal-measure specimens afford. 



The following are the measurements of Professor Prestwich's specimen (PI. XXXI, 

 fig. 6) : 



Greatest breadth of head-shield to extremities of genal spines 

 „ length of head-shield 

 ,, breadth of thorax 

 „ length of thorax 

 „ breath of axis of thorax 

 Length of telson .... 



[Another form of Limulus {Prestwichia Dana) from the Coal-measures, of Illinois 

 U.S., has been described by Messrs. Meek and Worthen, 1 for which Mr. Meek has since 

 proposed the generic name of Euproops, in allusion to the anterior position of its eyes. 3 



1 ' Geol. Surv. Illinois,' 1866, vol. ii ; 'Paleontology,' p. 395, pi. xxxii, fig. 2. 



2 'Geol. Mag.,' 1867, vol.iv, p. 320. 



34 



n 



; inches 



9 



lines. 



n 



inch. 



ii 



lines. 



3 



lines. 



7 



lines. 



