CANADIAN FOSSILS. 55 



obscurely five-sided, rounded at the apex, abruptly trunfcated at the 

 base ; mouth large, oval, without valves, situated w^ith its upper 

 margin about the centre of the length of the body ; ambulacral 

 orifice small, situated in the centre of the apex, a small pore near it 

 on the right hand side ; arms five, four of them extending down the 

 sides to the base, the fifth only two or three lines in length ; 

 thirteen pectinated rhombs ; column short, tapering to a point, 

 annulated by alternately wide and narrow joints, the former of 

 which are striated on their projecting edges in the longitudinal 

 direction. 



In this species the basal and second series of plates are pretty 

 regular, but the third series 'contains two plates, which are very 

 small in proportion to the others, an irregularity compensated by a 

 corresponding enlargement of two of the plates of the fourth series, 

 which are of so great a size that they extend from the top of the 

 body down to the second series, and thus fill up the blank in the 

 third series formed by the deficiency in the size of the two small 

 plates mentioned (see pi. iii. fig. la). The whole of the upper half 

 of the test presents very little of order in its structure, in consequence 

 of this disproportion in the size of the plates. 



The distribution of the pectinated rhombs is as follows : — 



On the anterior side there are two rhombs, a small one just below 

 xhe mouth on the right side and a large one above, which extends 

 from the mouth nearly to the apex (see pi. iii. fig. dj. 



On the right side there are two ; a small one near the apex and a 

 large one below it, but still nearly altogether in the upper half of 

 the fossil. Both of these are a little oblique, the large one with its 

 upper extremity leaning forward, and the small one leaning back- 

 ward (see pi. iii. fig. c). 



On the posterior or dorsal side there are four, two at the base, 

 one-half of each being on the basal plate of this side, and the other 

 half on the contiguous plate of the second series ; a third very small 

 rhomb is situated between the two small plates of the third series, 

 and a fourth very large one is divided between the two large plates 

 of the fourth series in the upper half of the fossil (see pi. iii. fig. F). 



On the left side there are five, a large one next the mouth, and at 

 its4upper angle another, which extends across the sides, sloping a 

 little downwards, with a third which rises nearly perpendicularly 

 from the posterior angle of the second one ; below this there is a 

 half-rhomb, and above the large one first mentioned in this division 

 a very small rhomb, only seen in perfect specimens (see pi. iii. fig. E). 



