198 Dr. Ivor Thomas — TrUohite Fauna of Devon and Cornu-all. 



about half-way between the axial furrows and the base of the genal 

 spine. 



The free cheeks are triangular, fairly elevated, and slope rather 

 steeply outwards to the marginal border. The bounding marginal 

 and occipital furrows meet at an angle of about 60°. The eyes are 

 prominent and sub-crescentic. They are situated on the summit of the 

 free cheeks near the glabella, extending to almost half the length and 

 rising nearly to the height of the glabella. The facets appear to be 

 circular, minute, and very numerous. 



The thorax is nearly twice the length of the head-shield, and 

 consists of nine segments, with a prominent, elevated, and rounded 

 axis very gradually tapering posteriorly. This axis is about four- 

 thirds the width of the pleural portion. The axial furrows are 

 well marked. The pleura run out at first practically at right angles 

 to the axis, and are then deflected backwards at the fulcrum, situated 

 about two-thirds of the width of the pleural portion from the axis. 

 Each pleuron is divided into two parts by a furrow. The anterior 

 part is deflected backwards gradually at the fulcrum, and overlaps the 

 posterior and less deflected portion near the margin. The state of 

 preservation of some parts of the thorax makes it impossible to give 

 greater details. 



The pygidium is semicircular and about five-eighths the length of 

 the thorax. The axis is prominent, convex, and rapidly tapers to 

 a blunt extremity. Seven rings can be clearly seen, while one or two 

 practically obsolete ones may also exist. The lateral lobes are too 

 poorly preserved for clear discrimination. The marginal border is 

 very well developed, its width at the posterior extremity being about 

 two-fifths the length of the pygidium. A gradual narrowing of this 

 border takes place towards the anterior terminations, and its surface 

 retains traces of ornamentation by furrows parallel to the margin. 



Locality and Horizon. — A single specimen of this trilobite was 

 obtained in the Upper Devonian of South Petherwin, Cornwall, and 

 was collected in an old quarry on the eastern side of the road at 

 Landlake. This exposure seems, according to Mr. Clement Eeid, to 

 have yielded all the best of the Petherwin fossils ; the quarry has now 

 been filled up with town refuse from Launceston. 



Remarks. — The two forms which show most resemblance to this 

 species ave Froetus nevadce, Hall (7, p. 219, pi. xxiii, fig. 19), from 

 the lower horizon of the Devonian limestone of Comb's Peak, Eureka 

 District, [Nevada, and Proetiis super stes, Barr. (1, pi. xv, figs. 5-9), from 

 etage H. Compared with the former, our species is less elongated, has 

 a narrower border of the head-shield, possesses much longer genal 

 spines, a less number of thoracic rings, and a considerablj^ broader 

 marginal border and more tapering axis of the pygidium. 



Compared with Froetus superstes, the Cornish species appears to be 

 more elongated, has a more curved posterior margin of the head-shield, 

 possesses smaller eyes, much longer genal spines, slightly diff"erent facial 

 sutures, a more elevated axial region of the thorax and pj-gidium, and 

 a considerably broader marginal border and blunter axis of the pygidium. 



The specific name dimhevidensis is derived from Dunhevid, the old 

 British name for Launceston. 



