244 MINERALS AND GEOLOGY 



IV 

 EDRIOASTERIDA. 



This class, entirely extinct and Palaeozoic, comprises a very 

 limited number of representatives. These apparently connect the 

 cystideans with the star fishes. They are stemless, circular, de- 

 pressed forms, varying from about a quarter of an inch to a little 

 over an inch in diameter. The under side of the body is unknown. 

 The upper side carries a five-rayed ambulacroid star, composed of a 

 double series of interlocking plates. The rays are curved in some 

 forms, and straight in others ; and are entirely confined to the upper 

 surface of the body. The margin of the disciform body is covered 

 with very small imbricating or partially-overlapping, scale-like plates. 

 The other parts or inter-ambulacroid spaces are protected also by 

 imbricating plates, but of somewhat larger size; and a " pyramidal 

 orifice," resembling that of a cystidean, is situated in one of these 

 inter-radial spaces. 



The principal genera comprise : (1) Agelacrinites, with curved rays, 

 like the " arms " of many ophiurian star-fishes ; and (2) Hemicystites, 

 with short, straight rays.* Species of both genera occur in our Silu- 



* The generic name Agelacrinus (now more commonly written Agclacrinites) was given by 

 Vanuxem to these forms, from the Greek, a-yeAij, a herd or crowd the first found examples 

 consisting of several individuals heaped or crowded together. But this condition of occurrence 

 is purely accidental. It was also thought that these forms, although without a stem, were 

 always attached to shells or other submarine objects by their broad base ; and this idea is still 

 retained by many writers. Hence the term Edrioasterida of Billings, from eSpaioo- fixed, 

 sessile. Examples are found now and then attached to fossil shells, but that condition is by no 

 means general. Out of fourteen or fifteen examples belonging to several species examined by 

 the writer, only one occurred in contact with a brachipod shell, and accidental contacts of that 

 kind are common among fossil bodies generalty. The structural characters of the Edrioas- 

 terians are still very imperfectly known. The mouth for instance, is almost universally regarded 

 as lying in ttie centre of the ambulacroid area at the summit of the disc. In a communication 

 published in the Canadian Journal, and in the Annals of Natural History, so long ago as 

 1860, the writer strove to maintain that this was not its true position, but that it was to be 

 looked for, as in ordinary asterians, &c., in the centre of the disc, below. No little support 

 seems to be lent to this view, by the subsequent discovery by Wyville Thompson of his 

 Echinocystites, regarded by him as a transitional type between cystideans and echinida. The 

 body is covered (apart from the ambulacra, &c.) with imbricating, irregularly arranged plates ; 

 the mouth is central, on the under side of the body ; and the anal opening, with protecting 

 pyramid of plates, is interradial in position (Edinburgh New Philosophical Journal, 1861). 



A magnificent specimen of Agelacrinites probably the best in Canada, if not on this Conti- 

 nentis in the collection of Dr. Grant of Ottawa. 



