NEOLIMULUS FALCATUS. 235 



it from the cheek, but it approaches to and unites with the border of the head-shield in 

 front. A line (which may correspond with the facial sutnre in Trilobites) passes from 

 the latero-posterior horns of the head-shield, obliquely across the cheeks, and unites with 

 the lateral border of the glabella just where the compound eyes are faintly indicated. A 

 raised V-shaped lobe, having its apex directed backwards, marks the centre of the 

 glabella ; near its apex four minute dots are visible under the microscope ; these are most 

 probably the larval eye-spots or ocelli. From the sides of this V-shaped lobe two corre- 

 sponding lines diverge and unite with the posterior border, whilst the lateral ridges 

 marking the semicircular border of the glabella curve inwards on approaching the 

 posterior border of the head-shield and unite with the median lines near the ocelli. The 

 segments succeeding the head-shield are free, and are strongly trilobed, the ends of the 

 pleurae are all distinct and falcate. The first six segments are reckoned as thoracic ; the 

 abdomen probably consisted of three segments, followed by an ensiform telson, but only two 

 of these segments are preserved. The breadth of the segments diminishes rapidly back- 

 wards, the first thoracic measuring five lines across, whilst the sixth is only three lines, 

 but the depth of each segment is nearly uniform. The form which appears to possess 

 the same number of segments is the genus Hemiaspis from the Lower Ludlow of 

 Leintwardine, in which we find six thoracic and three abdominal segments ; but with 

 this exception our Lanarkshire fossil is a much more Limuloid form than Hemiaspis^ 

 reminding one strongly of Mr. Baily's Bellinurus regina from the Irish Coal-measures ; 

 from this latter, however, it is also easily separable, both in its general form and also in 

 the non-anchylosed condition of the abdomen. From the genus Prestwichia it is also 

 separated by the free condition of all its segments. 



Not being able to refer the Lanarkshire specimen to any previously described genus 

 of Xiphosura, I propose to name it Neolimulus falcatus} 



If any one fossil Limulus gave promise more than any other to aid the palseontologist 

 in connecting the Xiphosura with the Trilobita ancestrally, it would seem to be the 

 one before us. Its great geological antiquity, as compared even with the Limuli of the 

 Coal-measures, naturally leads one to anticipate the discovery of some points indicative of 

 a more generalised type of structure than those of later date. The facial suture in the 

 head-shield, however, seems to be the only approach to a more Trilobitic character than 

 other Limuli present. 



Of the caudal termination of the body we know nothing, nor are we aware of the 

 form or number of the appendages. Its general facies is certainly Limuloid, not 

 Trilobitic. Nevertheless, the greater freedom of the thoracico-abdominal segments seems 

 clearly to mark it as a more active and natatory type than its adult modern representa- 

 tive, and rather to recall the free-swimming young of Limulus polyp/temus as it emerges 

 from the egg. (See PI. XXXIII, fig. 12.) 



1 veos, young, in allusion to its size, and also its early appearance in time (and limulus) ; and 

 falcatus, from the sickle-like form of the hody-segments. 



