SEA-UIICIIINS. 



457 



reduction is initiated, wliich is maintained and even accelerated 

 in Discoidea, until in J). c)/lindrica, and also in (fomdus, the peri- 

 stome has only nbout one ninth the diameter of the test. 



In dealing with this character it is important to realise that the 

 size of the peristome is not of necessity directly connected with 

 the presence or absence of jaws. It is true that in Galerojyygus 

 and Pyrina, where jaws were absent (at least, in adult forms), the 

 peristome is quite small ; but in the majority of the Olypeastroids 

 the peristome is smaller in proportion than in these genera, and 

 yet powei'ful jaws are present. 



In the matter of the proportional i-epresentation of the ambu- 

 lacral and interambulacral areas on the peristome margin, a slight 

 ])ut important change occurs in the course of the development of 

 the group. In Pygaste.r sens, str., the proportions are 1 to 1"4 in 

 favour of the interambuLicra,. In Comdus there is no appreciable 

 difference in the share taken by the two areas. This change is 

 partly due to the increased phyllodal tendency of the adoral parts 

 of the ambulacra in Comdus, but still more to an actual narrowing 

 of the interambulacra. In view of the extreme reduction which 

 is found in the latter areas of Clypeaste?' and its allies, the pro- 

 gressive change, though slight, is significant. 



Apart from the characters of the branchial slits, Avhich will be 

 considered in the next paragraph, the shape of the peristome 

 undergoes no important changes until Comdus is reached. In 

 that genus the circularity of its outline becomes slightly modified 

 into an elliptical shape, with a tendency for the long axis of the 

 ellipse to be oblique in its relation to the antero-posterior diameter. 

 The departure from the circular form is very insignificant in itself, 

 but when viewed in the light of the persistently elliptical, and 

 usually oblique, peristome of the Echinoneidfe, it becomes invested 

 with greater meaning. 



2. llie Branchial Slits. 



All of the genera iohat I include among the Holectypoida 

 possessed external branchiie. Within the boundaries of the group, 

 however, it is possible to trace the gradual reduction of these 

 structures (as indicated by the shallowing of the peristomial slits 

 through which they passed) until they become hardly appreciable 

 in size. In no case are there signs of the smooth calcareous 

 developments of the intel^ambulacra, for the suppoi-t of the gills in 

 a recTimbent position, which are characteristic of many of the 

 Regularia Ectobranchiata. The slight modifications of the plate 

 surface that exist are discussed in the section on the interambu- 

 lacral areas. 



In Pygaster sens, str., the branchial slits are extremely well 

 marked (in Megapygus they are slightly shallower), and the depth 

 of their incision renders the outline of the peristome festooned 

 and decagonal. This stellate shape of the peristome is retained, 

 though to a reduced degree, in Holectypus. In Discoidta the slits 



