14 Herbert L. HatvJcins — Studies on the Echinoidea. 



"false ridges" due to requirements of mechanical strength and to 

 the phenomenon of "super-calcification". The girdle, apart from 

 such modifications, is shown to be essentially primitive — more so 

 than that of any modern Diademoids. 



By the default of the proximal interambulacrals, or by an actual 

 transgression of the processes, the ridges are wanting in the 

 Clypeastroida, and the processes converge towards the interradial 

 line, often fusing in pairs. Such a character may readily have been 

 derived from the Holectypoid girdle. The presence of so great 

 a complexity of internal buttresses in the Clypeastroida points to 

 their derivation from some Cretaceous representative of the 

 Holectypoida, in which series alone such structures are strongly 

 developed. 



The NucleoUtes series of Jurassic so-called Cassiduloida (here 

 styled Nucleolitoida) seem to possess no trace of a perignathic girdle, 

 even in the simpler and early forms. The sudden disappearance of 

 the apparatus (there can be hardly any doubt that the ancestors 

 of all Euechinoida were gnathostomatous) seems to point to the 

 conclusion that the I^ucleolitoida arose independently of, but con- 

 temporaneously with, the Holectypoida. 



The Cassiduloida, to judge from the characters of one of the least 

 specialized forms, Conulopsis (and, I believe, from those imperfectly 

 known in Cassidulus itself), have a much thickened peristome in 

 which there is a strong resemblance to that of Conulus, although the 

 actual ossicles of the girdle are Avanting. It is suggested that the 

 false ridges are retained in this group, which becomes thereby 

 affiliated to the Cotiulus series of the Holectypoida. 



The Cretaceous Spatangoida have much thickening of the 

 peristomial plates, especially in the anterior region, but it has not 

 been possible to correlate any of their structures with the girdle, 

 true or false, of the Holectypoida. The oesophageal support 

 developed on one or both sides of the peristome in the later Heart- 

 Urchins is regarded as an entirely secondary structure, with no 

 aflinity to any portion of a perignathic girdle. 



EXPLANATION OF PLATE 11. 



Fig. 1. — Interior of peristome of Conulopsis abbreviata. x 6. The floor of 

 the test is thickened in the same manner as that of Conulus, and deep 

 slots (? for the reception of retracted buccal plates) are cut in the 

 peristomial " escarpment ". The actual aperture is vei'y slightly elliptical. 



,, 2. — Interior of peristome of Echinocorys vulgm-is. x 5. Except in 

 area 5 the floor of the test increases in thickness towards the peristome. 

 In the four lateral interambulacra single, erescentic hollows excavate the 

 " escarpment". The horns of the crescents in areas 2 and 3 are often 

 nodular, being the "auricles " noted by Klinghardt. 



,, .3. — Plan of interior of peristome in EcJiinocardiuin cor datum. In area 1 

 there is a small knob which rises partly from the neighbouring ambulacral 

 plates. In area 4 a long, twisted mesentery-support rises from foundations 

 exactly similar to those in area 1. The edge of the labrum is slightly 

 reverted. There is no special modification in areas 2 and 3. 



