OF CENTRAL CANADA PART IV. 247 



VII. 

 ECH1NOIDEA. 



The representatives of this class are the "sea-urchins " or " sea-eggs " 

 of popular parlance. They present a globular, disciform, oval, heart- 

 shaped, or other form of body, entirely enclosed in calcareous plates. 

 In a small number of both ancient and living genera, the plates are 

 overlapping, but in the great majority of echinids, they are joined 

 at their edges. These plates are of two general kinds : ambulacral 

 and inter ambulacral, respectively. The ambulacral plates, in all 

 echinida, form five separate, converging, linear or petaloidal areas, 

 each composed of two rows or zones of perforated plates. The inter- 

 ambulacral plates, lie also in five separate areas ; but in all Palaeo- 

 zoic types, with a single exception (Bothriocidaris), each area contains 

 more than two rows, commonly five or six ; whilst in all succeed- 

 ing with only two known exceptions (Anaulocidaris ? and Tetraci- 

 daris) these interambulacral areas contain two rows only.* The 

 perforations in the ambulacral plates give passage to delicate sucker- 

 feet ; and movable spines in some cases hair-like, in others club 

 shaped or cylindrical and comparatively large are borne on both 

 the ambulacral and interambulacral plates, more especially on the 

 latter, these being more or less distinctly tuberculated for the 

 support or attachment of the spines. The mouth, in echinids, is 

 always on the under-side of the body, and either central or sub- 

 marginal in position. The anal orifice is situated in some forms at 

 the apex of the shell or test, but in others it is at or near the under 

 margin. 



Echinida are very numerous in existing seas. They abounded 

 also in the seas of the Cainozoic and Mesozoic Ages, but appear to 

 have been rare in Palaeozoic times. No certain evidence of their 

 remains in the strata of Central Canada has yet been obtained : it is 

 unnecessary therefore in the present work to describe their leading 

 subdivisions and genera. 



* In the palaeozoic Bothriocidaris, the interambnlacrals form a single row. In Tetracidaris, 

 a Lower Cretaceous type, there are four rows of interambulacrals at the lower part of the test, 

 but these merge into the typical two rows at the upper part. 



