PHACOPS. 45 



Section. — Odontochile. 



If there were some doubts of the limits of the subgenus Chasmops, there can be none 

 at all of the general character of that which Emmrich called Balmania, and which name was 

 rightly changed to Odontochile by Corda, Dalmania having been in previous employ. 



Large expanded forms, with the glabella wider in front, and with the lobes nearly equal 

 or only gradually increasing forwards ; a depressed habit, and a caudal shield composed of 

 many segments (technically more than eleven), give a combination of characters easily 

 enough recognisable among the large Trilobites, which chiefly occur in Upper Silurian 

 rocks ; a few began in the Lower Silurian, a few extend into the Devonian ; but the bulk 

 of the subgenus are to be found in Wenlock and Ludlow rocks, and the genus is world- 

 wide. 



We have in this section the largest and most conspicuous of the Phacopida. And 

 while the section Acaste is chiefly Lower Silurian, and Chasmops wholly so, — the large 

 flattened species which form this group, with their numerous tail-segments, are almost all 

 Upper Silurian. They commence, so far as I know, in the May Hill Sandstone. The most 

 renowned of them all is the P. caudatus, and it is at the same time the most typical. 



I commence with those species in which the caudal portion is least extravagantly 

 developed: — 



Phacops (Odontochile) obtusicaudatus, Salter. PI. I, figs. 42 — 45. 



Fhacops obtusicaudatus, Salter, in Sedgwick and M'Coy's Synopsis Foss. of the 



Woodw. Mus., pi. i g, figs. 15, lfi. 

 — — Id. Mem. Geol. Survey, Dec. II, pi. i, p, 7, note. 



P. (Odont.) uncialis, capite elongato semielliptico tuberculato ; caudd brevi. Glabella 

 longa, lobo antico magno rotundato, lateralibus subaquis, radiatis, sulcis longis prqfundis. 

 Oculi elongati, antici. Gen<s angusta scrobiculata*., spinis modicis. Cauda trigona, apice 

 angulato, axe rotundato longo 12-anut/lato ; lateribus recti-sulcatis, sulcis 10, omnibus dupli- 

 catis ; margine angustissimo. 



This is known only in one locality ; but it is a common fossil there, and is accom- 

 panied by but very few other fossils. It is a gregarious species, occurring in crowds upon 

 the faces of the " bated " or cleaved rock in Coldwell quarry, Westmoreland. 



The head is half elliptical, frequently an inch and a half long, the width then being 



