448 H. L. Hawkins — Tubercuiation of Holectypoida. 



the adoral surface rapidly become large, and therefore few in number, 

 whereas they were peculiarly narrow on the adapical surface. 



The specimens of H. depressus from which Fig. 2, D was constructed 

 were all the large, Cornbrash forms. It will be seen that the 

 arrangement of the tubercles is of the normal Pygaster type. There 

 are, however, some features in the details of their structure which are 

 worthy of more detailed description. (See Fig. 1, H.) 



The proportionately greater size of the tubercles of the adoral 

 surface, and especially of their scrobicules, when compared with those 

 of the adapical region of the test, is more marked in this species than 

 in H. hemisphcericus, chiefly on account of the much larger area 

 occupied by each adoral plate. Apart from the extreme and Pi/yaster- 

 like size of the adoral scrobicules, the details of tubercle-structure are 

 alike in the two species. But on the adapical surface, on the plates 

 which are about midway between the apex and the ambitus (pis. 8-17 

 in the diagram), a curious feature, unique as far as any other species 

 I have examined are concerned, makes its appearance. 



In the line of the central row of primaries, and between the main 

 tubercle and the adoral margin of the plate, there occurs a small 

 tubercle sunk in a deep socket. The details of the scrobicule and 

 boss of this supernumerary tubercle seem identical in every way with 

 those of the main series, but, owing to the obliquity of the floor of the 

 socket, the tubercle points slantingly in an adoral direction. At first 

 sight the socket looks like the open pit of a splmridium, but, apart 

 from the fact that it is nowhere near the peristome, the presence in it 

 of a perfectly normal tubercle precludes the possibility of its having 

 this function. The other, and more feasible, suggestion which its 

 appearance provokes, is a comparison with the sunken, oblique major 

 tubercles of the interpetalous regions of some of the more highly 

 specialized Spatangids. The details of the relations and structure 

 of this peculiar feature will be understood more clearly by an 

 examination of Fig. 1, H, than from verbal description. I have not 

 found any trace of a corresponding feature in the H. depressus from 

 the Inferior Oolite. 



In addition to this anomalous series of additional tubercles, it will 

 be noticed, on reference to Fig. 2, D, that in the same region of the 

 test some of the granules rise to the importance of actual tubercles. 

 These extra tubercles, indistinguishable structurally (except in point 

 of size) from the ordinary serial primaries, are always developed on or 

 near to the central line of the plate, and to the adapical side of the 

 main tubercle. The presence of these hypertrophied secondaries is 

 again a precocious feature. In Discoidea and Comdus they are typical 

 and numerous, but the Cornbrash M. depressus is the only Oolitic form 

 in which I have observed them. 



6. Discoidea cylindrica (Lam.). 



(a) The Structure of the Tubercles. (Fig. 1, F.) 



The disparity in size between the tubercles of the adapical and 



adoral surfaces reaches its extreme (for the Holectypoida) in Discoidea. 



Not only are those of the upper surface small and inconspicuous in 



themselves, but a considerable increase in the number and size of the 



