H. L. Hawkins — Tuberculatum of Holectypoida. 445 



well marked. The boss rises in an. S-curve from the vertical edge of 

 the terrace. It is always in the centre of the scrobicule. The parapet 

 is but slightly raised above the platform, and both are smooth. The 

 mamelon is small in proportion to the size of the boss, but almost 

 covers the platform. It is roughly spherical, and has a small, circular 

 foramen. 



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



As in all the Holectypoida, the plate next the apical system has only 

 one tubercle, subcentral in position. In Pt/gaster the first four or five 

 plates may retain this primitive character. The tubercles of this first 

 series occur on every plate from apex to peristome, and, from the 

 second plate downwards, are all towards the adoral margin of the 

 plate in position, though never far from the centre. On the inter- 

 radial tract there appear, in a persistent sequence as the transverse 

 diameter of the plates increases, new tubercles, which develop into 

 horizontal as well as vertical lines. Their position is midway between 

 the adapical and adoral margins of the plates, a feature which 

 distinguishes them clearly from the excentric main series. On the 

 adradial tract there is a corresponding growth of fresh tubercles, but 

 these develop into a different order. The first formed adradial tubercle 

 is quite near to the adapical margin, while the second is equally near 

 to the adoral. This zigzag horizontal arrangement is maintained, 

 however many vertical rows of adradials may be developed. A study 

 of the diagram will render the arrangement more intelligible than 

 a detailed description. The disappearance of the rows of tubercles, 

 after the ambitus is passed, is definitely in the inverse order of their 

 appearance, on both the adradial and interradial tracts. 



This method of arrangement is the simplest and most typical of 

 those found among the Holectypoida. It results in an appearance as 

 of broken horizontal lines of tubercles in the middles of the inter- 

 ambulacra, and of massed arrays of oblique series along the adradial 

 regions. The tendency to a degeneration, or rather a lack of 

 development, of the large radioles in the adapical interradial tracts, 

 which is so prominent a feature in such genera of Regular Echinoids 

 as Ccelopleurus, is thus seen to be present in Pygaster. 



2. Pygaster umbrella, auct. 

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

 The primary tubercles are small and inconspicuous on the adapical 

 surface, except those round the periproct. Adorally they are 

 considerably larger, and increase steadily in size almost to the margin 

 of the peristome. The scrobicule is practically flush with the test- 

 surface adapically, and but little more excavate below. The scrobicular 

 ring is even less marked than in P. semisulcatus. The basal terrace 

 and boss are similar to those in that species, but smaller in proportion, 

 even adorally. The most significant feature of distinction between 

 the tubercles of the two species is that in P. umbrella the boss is not 

 in the centre of the scrobicule (in adoral tubercles), but tends towards 

 the adoral side of the centre. The excentricity is not great, but is 

 a persistent featui'e, and one which suggests a comparison with the 

 extreme excentricity of the bosses on some parts of the tests of 



