SOME FUNDAMENTAL TYPES OF HYDROSPIRES WITH 

 NOTES ON POROCRINUS SMITHI GRANT 



* BY GEORGE H. HUDSON 



In many of the limestone beds of Valcour island, deposited under 

 lagoon conditions on the lee side of reefs and containing much fine, 

 silicious or argillaceous material derived from land wash or atmos- 

 phere, we find an abundance of echinoderm plates so well preserved 

 as to show minute details of form and structure. This material is 

 so abundant and so varied in nature as to throw much light on the 

 paleobiology of the group in question and the value of this material 

 is enhanced by the fact that it represents a time so near the begin- 

 ning of available life records. It is the purpose of this paper to 

 point out four fundamental types of hydrospires, three of which 

 are shown in the Valcour island material. As one of these types 

 is identical with that presented by Porocrinus, and as this peculiar 

 type seems to be little known, we shall here use Porocrinus 

 s m i t h i to exemplify it. 



In plate i we present two hydrospire areas enlarged to lo 

 diameters and photographed without gum mounting, as this gives 

 us the more brilliant surface detail. In plate 2 we show a single 

 hydrospire area under gum and enlarged to 20 diameters. Billings 

 in Canadian Organic Remains, Decade IV, page 34, called these 

 "exposed hydrospire areas "pectinated rhombs," but the ontogeny 

 and position of the folds will show that they differ fundamentally 

 from a true pectinirhomb. In the latter the hydrospire is confined 

 to the margin of two plates, the folds are perpendicular to these 

 margins, the youngest folds lie nearest the plate corners and these 

 corners are left fairly strong with plate stereom and so fitted to 

 prevent undue pressure on the sutural ends of the hydrospire folds. 

 Hydrospires of this type occur in the Caryocrinidae and Callo- 

 cystidae. They are variously protected, being sometimes covered at 

 the suture as in Callocystis or more completely covered as in Cary- 

 ocrinus. In Porocrinus each hydrospire is situated at the junction 

 of three plate corners, the folds meet the sutures at an acute angle 

 (for they are parallel with radii running to plate corners), the 

 oldest folds lie nearest these corners and pressure on their elongated 

 and therefore weakened walls is prevented by a heavy epithecal bar 

 of stereom which crosses and strengthens the middle portion of 



