H. L. Hawkins — Tuberculation of Holectypoida. 447 



general level of the test. The difference in size between the tubercles 

 of the adapical and adoral surfaces is hardly appreciable, and they are 

 closely packed over the whole test. 



The principle of their arrangement is, however, exactly similar to 

 that in P. semisulcatus. 



4. Holecttpus HEMisPHiERicus (Desor). 

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



There is a very marked disparity in size between the tubercles of 

 the adapical and those of the adoral surface in this species, the former 

 being less than half the dimension of the latter. 



The scrobicule is circular and moderately excavate, but the scrobicular 

 ring is poorly developed. The basal terrace is even more abruptly 

 marked off from the floor of the scrobicule than in Pygaster. Its 

 diameter is but little less than that of the scrobicular circle, so that 

 the boss, which is fairly high, almost fills the scrobicule. This feature 

 is more noticeable adapically than adorally, since in the latter region 

 the area of the scrobicules is increased rather more than that of the 

 bosses. The mamelon is minute and perforated, but, in spite of its 

 small size, almost entirely covers the narrow, uncren elated platform. 



(b) The Arrangement of the Tubercles. (Fig. 2, C.) 

 The general plan of the tuberculation is the same as that observed 

 iu Pygaster semisulcatus, except that, in spite of the considerably 

 smaller size of an adult H. hemisphcericus, it normally bears more 

 vertical series of tubercles per plate than the larger Pygaster. The 

 specimen on which the diagram is chiefly founded shows a feature, 

 present in only a few of those that I have examined, which seems 

 significant. On the adradial tract of pi. 14 (marked with a cross in 

 the figure), it will be noticed that, in place of the two adapically and 

 adorally situated tubercles which go to the formation of the zigzag 

 arrangement, three distinct tubercles are present. Trifling though this 

 irregularity appears, it would seem (especially in view of its occurrence 

 in several specimens), to have some phylogenetic significance. The 

 triple arrangement of the tubercles becomes the rule in Conulus, 

 as will be seen later, and here, in the Inferior Oolite, there seems to 

 be a precocious tendency in some specimens of H. hemisphcericus 

 to assume characters which do not become general until the Middle 

 Cretaceous. 



5. Holectypus depressus (Leske). 



There seem to be two very distinct types of Holectypus passing 

 under this name, one from the Inferior and Great Oolite, and the other 

 from the Cornbrash. At present, however, I do not feel justified in 

 distinguishing the two forms by name, especially as this paper is not 

 systematic in purpose. The Inferior Oolite specimens are usually 

 small, and their tuberculation is practically identical with that of 

 H. hemisphcericus. There is one very notable feature which 

 distinguishes both H. depressus from the last-named species, this 

 being the relative number of plates above and below the ambitus. 

 A comparison of the two figures (2, C and D) will make the contrast 

 clear. In H, depressus, as soon as the ambitus is passed, the plates of 



