CANADIAN FOSSILS. 19 



could be brought together at all in such a manner as to constitute 

 an organic structure. By several fortunate discoveries, however, of 

 fragments of the body, with some of the plates still occupying their 

 natural position, we are now in possession of data by which the 

 more important details can be demonstrated, and no doubt other 

 specimens will in course of time be found to supply the remainder. 



The remains of this species most usually occur in an extremely 

 comminuted state, and in such abundance that in many localities 

 they constitute almost the whole of the ingredients of beds of solid 

 rocks, of from six inches to three feet in thickness. Very frequently, 

 where the rock has been long exposed to the action of the atmos- 

 phere, the large unbroken plates are weathered out and displayed in 

 strong relief upon the surfaces of the strata ; and as they are easily 

 recognized and strictly characteristic of the Chazy limestone, they 

 constitute a most valuable and safe guide to the explorer in tracing 

 out the distribution of the formation, or in determining the age of 

 isolated patches where other fossils may be absent, or where the test 

 of superposition cannot be made available. 



The form of these large plates is nearly triangular, two of the 

 sides being straight or gently curved outwards, and the third more 

 or less strongly arched inwards. This last mentioned side, when "the 

 plate is in its natural position in the perfect cup, is the lower side, 

 and the angle immediately opposite to it the apex. The plates are 

 not flat, but have a concavity extending from the apex downwards, 

 being the deepest at the base, thus giving a form somewhat like that 

 of an ordinary shoe- lifter. The length of the base measured in a 

 straight line between the two lower angles, and, without following 

 the curvature, is, in a specimen of the average size, about ten lines; 

 the length of the other two sides, when compared with the base, is 

 either equal to it or a little greater. The surface is ornamented with 

 fine straight parallel striae, from six to ten in the width of a line, and 

 running in a direction from the base upwards. When the specimens 

 are a little worn these strige are obliterated, and the surface is then 

 smooth or granular. The concave side of the plate is the outer side. 



The specimen represented by figures h', Ik and 11, Plate I., shews 

 that the upper or ventral surface of the perfect fossil is divided into 

 five equal portions by five rays or pseudambulacra, which radiate 

 from the centre outwards, and that the place of the triangular plates 

 above described is in the angles between the rays. There are thus 

 five of these plates in the skeleton of each individual. The rays 

 consist of two rows of narrow, elongated plates, placed transversely, 



