448 H. L. Hawkins — Morphology oj Echinoidea. 



system. In France, on the other hand, the genus ranges well into 

 the Upper Chalk (Senonien), but occurs only in those districts where 

 the deposits are full of detrital matter. In this respect it resembles 

 most of the Holectypoida (excluding Discoidea and Conulus), and the 

 deduction is surely sound that it was a genus confined to the littoral 

 tracts of the sea. The extremely close correspondence in structure 

 between Pyrina and Echinoncus thus seems associated with a 

 similarity in habitat. Co?iulns, on the contrary, was certainly not 

 restricted to shore-lines, or even to shallow water. The distribution 

 of the recognized English species is particularly instructive. 

 C. rhotomagensis, which occurs in the Selbornian, is superficially like 

 a typical Pyrina, both in ambital outline and relative height. 

 C. castanea, from the Lower Chalk and lower parts of the Middle 

 Chalk, is still elongated, but shows almost Globator proportions 

 in height. C. subrotundus is low and roundish in the zone of 

 P. cuvieri (save for very large, almost globular, specimens, probably 

 representing a distinct species, and here called C. sp.) but grows 

 surprisingly tall in the zone of Terebratulina. C. albogalerus is 

 relatively low (though markedly conical) in the lower zones of the 

 Upper Chalk, but becomes exceedingly high, especially when large, 

 towards the top of the zone of M. coranguinum. C. rhotomagensis 

 is definitely a Conulus, but its "Pyriniform " characters suggest, not 

 merely that it may have descended from Pyrina, but that it was 

 living under conditions similar to those surrounding that genus. 

 The two horizons at which species of Conulus attain their greatest 

 height are those that seem to mark the deepest conditions that 

 obtained during the dominance of the Chalk-sea over Britain ; while 

 the comparatively depressed forms from the lower parts of the Upper 

 Chalk can be correlated with the Chalk- Rock uplift. The tendency 

 for Echinoids, and other organisms, to become vertically elongate in 

 deep water has often been remarked, and there seems every reason to 

 consider Conulus as a clear illustration of that phenomenon. But for 

 the present purpose it is important to recognize that Conulus departed 

 morphologically from a Pyriniform character progressively as it 

 adapted itself to a changing habitat. The reduction in size, and 

 increase in numbers and irregularity of distribution, of its radioles 

 invite comparison with the bristle-like nature of the radioles of most 

 Irregular Echinoids that dwell in sand or ooze ; while the elaboration 

 of the tube-feet nearest to the peristome also points to an analogy 

 with the phyllodes of the Echinolampidaa or the "pseudophyllodes " 

 of Spatangoids. A logical deduction from these points is that, if 

 Conulus had remained in the littoral zone, it would have kept the 

 essential shape and characters of Pyrina, while its periproct, in 

 obedience to the principle that seems always to influence that 

 aperture, would have become first marginal (as, in fact, it did), and 

 then infra-marginal. In brief, it would have developed into a form 

 very much like Ecliinoneus. But by its desertion of the shore, and 

 its preference for the oozy depths of the open sea, it underwent 

 modifications that led it far away from the Ecliinoneus direction, and 

 that produced more than a superficial likeness to the Echinolampas 

 series. 



