Dt Traquair on the Genus Gyracaiithus (Agassiz). 101 



entire, its length could not have been less than 2 feet. 

 Another, wanting the point, must have been about the same 

 size ; and fraj^ments are not uncommon which indicate still 

 greater dimensions. The general form is elongated and 

 slender, the breadth increasing more rapidly towards the base 

 in adult specimens. They are very variable in respect of 

 curvature : in some both antero-posterior and lateral curves 

 are well marked ; in others the lateral bend is only slight or 

 hardly perceptible ; and I have one which appears almost 

 perfectly straight in doth directions. Every one of them, 

 without exception, is nevertheless asymmetrical as regards 

 those special points of configuration upon which I have 

 dwelt in connection with G. tuherculatus, and, as in that 

 species, they may be arranged in pairs. 



In the form of tlie non-sculptured inserted part, with its 

 posterior sulcus, and in the general configuration of the spine 

 as seen in transverse sections, G. nobilis closely resembles G. 

 tuherculatus. The posterior marginal keel is in its distal 

 portion strongly denticulated ; in one specimen the denticles 

 may be traced, from the point, a distance of 10 inches in the 

 direction of the base. The posterior groove varies much in 

 its degree of sharpness ; in some it is very shallow and 

 slightly marked till towards the point, while in others it is 

 very well defined along its whole extent. In adult specimens 

 continuations of the gyrating ridges usually encroach upon it 

 at its commencement ; but the salient point in this species 

 lies in the disposition and mode of tuberculation of these 

 ridges. At the proximal end of the spine, in adult examples, 

 they are disposed much as in G. ticherculattis, meet each other 

 anteriorly at much the same angle, and are closely tubercu- 

 lated along their whole extent. But near the closure of the 

 sulcus this close tuberculation becomes limited to the anterior 

 aspect, each ridge as it arises and advances forward showing 

 first a comparatively distant tuberculation, then a smooth 

 space (sometimes very minutely crenulated) on the side of the 

 spine, and finally becoming thick and coarsely tuberculated 

 as it turns round to the front. Where this feature of the 

 ridges commences they also become excessively oblique and very 

 delicate, and in some specimens they also occasionally bifur- 



