Herbert L. Haivkins — Studies on the Echinoidea. 13 



particularly to the anterior and posterior edges of the aperture. 

 The thickness is always greater in the interambulacral parts than in 

 the ambulacral. Save on the edge of the labrum, there is no 

 indication that the thickened margin is adapted to any special 

 requirement beyond that of strengthening the free edge of the 

 coroua. In the case of area 5, the lip is often rolled over away from 

 the peristome, and overhangs the "bowl" of the spoon-like labrum 

 to a slight extent. In the (presumably) more primitive types, where 

 the labrum is hardly worthy of the name (e.g. Echinocorys) there is 

 no trace of such a specialization of the margin, which is merely 

 thickened similarly to, but in a less degree than, the other inter- 

 ambulacral edges ; it must consequently be a secondary development 

 suited to the needs of a labrum, aiid can have no homology with the 

 perignathic ridges of less specialized groups. On the thickened 

 margins of areas 2 and 3, in Echinocorys and Micraster, the line of 

 attachment of the buccal membrane is often apparent. Within this 

 the secondarily thickened margins rise almost vertically, and seem to 

 show no feature inconsistent with the belief that they are simply the 

 truncated edges of the coronal plates. They certainly constitute 

 "false ridges", but whether they are to be correlated with those of 

 the Holectypoida is very doubtful. 



In most of the Kecent Heart-Urchins there is a more or less 

 extensive alar projection from interambulacrum 4 near the margin of 

 the peristome. In many forms a corresponding, but only just visible, 

 projection occurs in area 1, These projections afford support for 

 the mesenteries holding the proximal parts of the alimentary' canal. 

 They have been regarded by some authors as modifications of parts of 

 a perignatKic girdle. The only fossil in which I have succeeded in 

 recognizing even a trace of such a projection is a Schizaster from 

 the ? Miocene of East Africa. The extreme delicacy of the free part 

 of the structure is such that only its foundations could be hoped for 

 under the conditions of fossilization. It is possible that the pre- 

 sumed "auricles" described by Klinghardt in Echinocorys may 

 represent such a structure, though they seem to be somewhat sporadic 

 and irregular in their occurrence. But I have never seen a trace of 

 these projections in any Cretaceous Spatangoid. As far as my 

 experience goes, these CBsophageal supports are a recent development 

 restricted to fully specialized Heart-Urchins, and so are not likely 

 to be homologous with any part of a true perignathic girdle. 



As at present known the peristomial characters of the Spatangoida 

 give no satisfactory clue to their morphogenetic relationships, while 

 their more elaborate features are purely secondary developments, 

 quite uuconnected with any ancestral qualities. 



7. Sdmmaky. 



The perignathic girdle of the Holectypoida is believed to consist 

 of disjunct processes situated almost on the adradial sutures, with 

 minute " true ridges " occupying the inner surfaces of the unpaired 

 interambulacral plates. This character is constant throughout the 

 group, although it may be partly obscured by the development of 



