ON THE OCCURRENCE OP ASAPHUS MEGISTOS. 141 



(Transactions of the Association of American Geologists and Natural- 

 ists : Boston, 1843), chiefly in the more acute outline of the head- 

 shield, and in the development of the glabella. The latter character, 

 however, in the genus Asaphus, is so indefinite and irregular as to he 

 of little importance ; and the agreement betvpeen the two forms, in all 

 essential characters, is too close to admit of their separation. On 

 comparing these examples with various fragmentary specimens in our 

 possession, we have reason to believe that A. tnegistos occurs in the 

 Trenton Limestone of other parts of Canada, as well as in that of 

 Cobourg. As we propose shortly to publish a figure and revised 

 description of this species in a detailed Monograph on the genus 

 Asaphus as occurring in Canada, we will merely state, at present, that 

 A. megistos is distinguished from other species, by the possession of a 

 smooth pygidium, coupled with the presence of short narrow horns at 

 the posterior angles of its head-shield. Apart from the horns, it 

 much resembles in its general aspect the well known Asaphus platy- 

 cephalus. The branches of the facial suture, as in the latter form 

 and other species of the same type, meet in a well defined point close 

 to the anterior margin of the head-shield. 



In the specimen discovered by Mr. Blackwell, we find some of the 

 pleurse broken away, and the stone retaining a sharp impression of the 

 under side of these. In this impression there occurs on each pleura, 

 near the end and close to the upper margin (exactly as in ^. Hincksii : 

 Canadian Journal, vol. iv., page 3), a single deep and oblique pit, 

 somewhat triangular in form, and with the deeper part towards the 

 posterior extremity of the body. These peculiar cavities are situated 

 just where the under part of the shell or its incurved portion termi- 

 nates. This we have verified by actual observation. One would 

 naturally expect to find a projecting point or tubercle at the spots in 

 question, but we have failed to detect anything of the kind. On the 

 contrary, there appears to be a hollow space or cavity at these places, 

 filled with the substance of the enclosing rock. Hence the pits or 

 indentations must have been made by (or moulded upon) some soft 

 or perishable organ ; and this seems the more probable, as the pleurae 

 have anteriorly a broad and flattened surface, admitting — as shown by 

 one of the specimens which is partially rolled up — of very complete 

 imbrication, and yet no trace of a cavity or corresponding mark of any 

 kind is to be seen on these surfaces. At the same time, it must be 

 observed that similar cavities occur in the impression of the head- 

 shield, one on each side, near the posterior angles ; and in the specimen 



