12 Herbert L. HawJcins — Studies on the Echinoidea. 



to their proximal termination, and are then steeply bevelled off by 

 a pair of concave bays or " combes", between which the areas project 

 as rounded spurs almost overhanging the slope of the " escarpment ". 

 The two "combes" in each area are most deeply excavated just on 

 the interradiad side of the adradial sutures. Here they appear as 

 deep slots, differing only in proportion from the hollows in the false 

 ridges of Conulus. By analogy, it would seem that these hollows in 

 the peristomial margin of Conidopsis must have been destined to 

 receive massive buccal plates when the mouth was opened. 



Unless Comdojjsis is on a side line of evolution, it would appear to 

 be a simple member of the Caratomidae, and as such more or less 

 ancestrally related to the ^chmolampas-gronp. Whatever be its 

 other affinities, it must surely be nearly related to Conulus, to judge 

 from the nature of most of its test-structures. As far as tlie 

 perignathic girdle is concerned, it may be considered to show the 

 retention of the false ridges of Conulus, after the loss of both sets of 

 the "true" elements of the girdle. From the scanty evidence at my 

 disposal, I believe that Catopygus, a far more "advanced" type of 

 Cassiduloid, has a similar perignathic structure. Clarke & Twitchell 

 {The Mesozoic and Cenozoic Ecliinodermata of the United States) give 

 figures of internal moulds of Cassidulus californicus (pi. Ixv) and 

 Pygorhynehus gouldii (pi. Ixxix) wliich show a strong interradial 

 thickening of the interior of the test at the peristome in these 

 typically Cassiduloid species. 



6. The Spatangoid Peristome. 



For the present purposes I have examined the interior of the test 

 in many examples of Echinocorys, Micraster, and Echinocardmm. 



The predominant feature of the peristome of the Spatangoida is its 

 progressive adaptation to the requirements of a burrowing life and 

 an "earthworm" mode of feeding. The part of the aperture 

 bounded by interambulacrum 5 comes to project below the general 

 level of the adoral surface, and its margin develops into a spoon-like 

 labrura. All Spatangoids, as far as is known, are entirely destitute 

 of jaws. MacBride has recognized as teeth certain spicules produced 

 at a very early stage of post-larval ontogeny, but no pyramid or 

 auricular vestiges seem to be associated with them, and they disappear 

 shortly after their formation. 



Klinghardt has identified as "auricles" certain protuberances at 

 the side of the peristome in Echinocorys, but the examination of 

 considerable numbers of specimens of E. vulgaris, representative 

 of many of the later growth stages, has convinced me that these 

 blunt excrescences are not processes (being based upon the inter- 

 ambulacra), so that they can hardly be homologous with any part of 

 a perignathic girdle in the strict sense of the term. 



Reference has been made in the preceding article of this series to 

 the curious "mode" of excessive stereom-formation prevalent among 

 the Kchinoids of the Cretaceous period. The early Spatangoids 

 almost universally adopted this character, so that the margins of 

 their peristomes are always of considerable thickness. This applies 



