446 H. D. Hawkins — Morphology of Echinoidea. 



transversely elongate, and by their meeting form the entire posterior 

 margin of the system. The proportions and disposition of the nine 

 plates are practically identical in all three genera. 



Ambulacra. — Narrow and lanceolate, with narrow pore-fields and 

 relatively wide, thickly ornamented, perradial tracts. Pores similar 

 throughout, constantly minute ; completely uniserial except near the 

 peristome in Conulus. Plates grouped in threes, comparable with the 

 "Echinoid" type of triads (Duncan), practically throughout the 

 areas. There is never more than one demi-plate in a group, however 

 reduced in height the smaller primary may become. Ornament 

 similar in character to that of the interambulacra, very regularly 

 distributed as far as concerns the main tubercles. 



Interambulacra. — Relatively broad, built of fairly low, usually bent, 

 plates thickly covered with ornament. Primary tubercles well- 

 scrobiculate (especially near the ambitus and adorally), arranged in 

 a definite sequence. The central series in each column is not sensibly 

 different from the rest except in position. Adradially and interradially 

 the other series are developed in more or less distinct chevron 

 pattern. The tubercles are perforate in Pyrina and Conulus, 

 imperforate in Echitioneus. Smaller ornament profuse and very 

 diverse in character, always including " glassy tubercles". 



Excluding variation in the shape of the test (which is probably due 

 to the influence of surroundings and consequent mode of life), the 

 differences that exist between the three genera can be regarded as of 

 two kinds. The position of the periproct changes in a direct sequence 

 chronologically, since Pyrina is the first of the three to appear and 

 Echitioneus the last. The nature of the change is very suggestive of 

 evolution along a line parallel to that which led from Plesiechinus , 

 through Holectypus to Discoidea, and to that proved in the ontogeny 

 of several Spatangoids. Curiously enough, the other features of 

 difference do not seem to follow the same sequence. The perignathic 

 girdle of Conulus is more massive than that of Pyrina (though the 

 greater thickness of the test in the former may perhaps in part 

 account for this), while it is completely absent from the adult 

 Echitioneus. From Pyrina to Echitioneus the sequence is maintained, 

 but Conulus fails to take its place in the series in this respect. Again, 

 in the matter of the ambulacra, Conulus has strongly triserial (almost 

 phyllodal) pore-pairs near the peristome, while in Pyrina and 

 Echitioneus simplicity is retained throughout. In interambulacral 

 ornament Pyrina and Conulus agree in the perforation of the tubercles, 

 but all three genera are alike (save for details) in the order in which 

 the tubercles are grouped on the plates. It is interesting to note that, 

 by the often bewildering confusion of its tuberculation, Conulus is 

 slightly less " Holectypoid " than the other two genera, whereas its 

 differences from them in other points seem all to trend towards the 

 true Holectypoid characters. Regularity of tubercle sequence is 

 a diagnostic feature in the order, and is not met with in the adults 

 of any other kinds of Irregular Echinoids — that is, if Pyrina and 

 Echitioneus are admitted into the Holectypoid fold. It can always be 

 recognized in Conulus, but in some species, notably C. castanea and 

 large examples of C. subrotundus, it is very difficult to trace. 



