PRESTWICHIA BIRTWELLT. 247 



are the larval eye-spots ; the compound eyes are placed on its raised lateral border. 

 The glabella is divided down the centre for half its length by a raised ridge, which 

 terminates in a blunt spine posteriorly, whilst in front it bifurcates, forming an arched 

 ridge on either side of the mesial line. Two equidistant ridges, nearly two lines apart, 

 also spring from the posterior border on each side of the mesial ridge of the glabella, 

 and unite with and complete the arched ridges in front. Two lesser arches unite the 

 median and the two next lateral ridges, half way up on either hand. The axis of the 

 thoracic somites is narrow ; each somite is ornamented on the median line with a single 

 blunt tubercle, a somewhat larger tubercle marks the coalesced and rudimentary 

 abdominal portion. The broad membranous margin of the thoracico-abdominal somites 

 is divided by seven strongly marked spines of nearly equal length, which do not, 

 however, project far beyond the broad membranous margin. The telson is unknown. 



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

 fig. 5) : 



Greatest breadth of head-shield . . .2 inches. 



„ length of head-shield . . .9 lines. 



Length of genal spines 1 . . . 2 „ 



Greatest breadth of thoracic somites . . .1 inch 7 lines. 



„ ,, axis of thoracic somites . . 3 lines. 



Breadth of thorax at point of union with head-shield . 1 inch. 



Length from back of head-shield to point of insertion of 



the telson . . . . .9 lines. 



From back of head-shield to edge of membranous border of 



abdomen . . . . .1 inch. 



Species 3.— PRESTWICHIA BIRTWELLI :— H. Woodw., 1872. PI. XXXI, 



figs, la — c. 



i &" 



Prestwichia Birtwelli, H. Woodio. Geol. Mag., 1872, vol. ix, pi. x, figs. 9 and 10, 



p. 440. 



For a knowledge of this small but well-marked and very characteristic species of 

 Coal Limulus I am indebted to the kindness of Mr. Thomas Birtwell, of Gawthorpe 

 Gardens, Padiham, Lancashire, who obtained it from the Coal-measures at the Cornfield 

 Pit on the south bank of the River Calder, near Padiham. It was from this pit that the 

 small fossil Arachnide, Architarbus subovalis, H. Woodw., 2 was obtained by Mr. 

 Birtwell. 



1 Length of genal spine in specimen from Kilraaurs 7 lines. 



2 See 'Geol. Mag.,' 1872, vol. ix, p. 385, pi. ix, figs. 1 a, b. 



