438 Herbert L. Hawkins — Studies on the Ecliinoidea. 



Conulus — the reverse would have been anticipated. Erorn Plesie- 

 chinas onwards the record of the perignathic girdle is one of ever- 

 increasing obliquity, and in the Clypeastroids the angle between the 

 plane of the adoral surface and a line joining the peristome-margin 

 to the top of the girdle is very acute. The reversal of the trend of 

 evolution in Conulus would thus seem to indicate that there is no 

 direct phyletic course from the Holectypoida to the Clypeastroida by 

 that route ; a conclusion that is supported by most other morphological 

 indications. But it is a reversion towards the " Regular" character, 

 and as such might be held to imply that the jaws of Conulus 

 "improved" in quality during ontogeny. On the other hand, it 

 seems equally logical to suggest that the continued thickening of 

 the proximal parts of the interambulacra prevailed over the failing 

 girdle, so that its increased slope is an expression of its waning 

 functional importance. On general grounds, I incline to accept the 

 latter alternative as the probable explanation of this anomalous 

 feature. 



(b) The Structure of the Girdle. (PI. XXVIII, Figs. 1 and 2.) 



The general characters of the girdle of C. albogalerus are well 

 known, and little in the way of description needs to be added to the 

 account given by Loven in Echinologica. The chief problem is 

 concerned with the interpretation of the observed structures. For 

 this purpose I have chosen the very large specimen to which 

 reference has been made in the preceding section, since this shows 

 all the details of the girdle with an exaggerated clearness. The 

 peristome is elliptical, with its long axis in the direction 3, I. 



The general plan of the girdle strikingly resembles that of a Tudor 

 rose. The five "petals" rise sharply from the peristome-margin in 

 concave and embayed escarpments. Outside them the interambulacra 

 rise to a slightly greater height, and then gently slope down towards 

 the ambitus. The " petals " are separated from one another by the 

 deep and narrow sulci of the ambulacra. The lateral margins of 

 a "petal" consist of slender, projecting processes, based upon the 

 bordering ambulacra, and visibly sutured to the main structure. 

 Each process culminates distally in an almost spherical and glossy 

 knob, which projects upwards and inwards beyond the general level 

 of its surroundings. The " petal " itself may be considered to consist 

 of three parts. There is a central prominence on the peristome 

 border, rising sharply and culminating in a bevelled crest. Above 

 this the chief part of the " petal" rises in a deeply concave slope, 

 bisected by a median carina, which, though prominent, is also 

 concave in the upward direction. At the top of the structure, 

 overhanging the bays and carina of the main surface, is an almost 

 flat area which slopes very gently towards the peristome, and has an 

 imperfectly denned rim distally. (This platform is practically con- 

 existent in smaller examples.) Beyond the platform the inter- 

 ambulacrum rises a little above the sulcate suture which bounds the 

 girdle, and then falls away gently towards the ambitus. The only 

 sutures that I have been able to detect with certainty are (1) those 

 between the bordering processes and the "petal", (2) a crescentic 



