H. L. Hawkins — Morphology of Echinoidea. 451 



the ambitus is rounded, and comes about midway between the apex 

 and the peristome, conditions are more favourable for a strict 

 application of the law. 



The diagram (p. 445) is practically self-explanatory, but a few 

 points will bear verbal emphasis. In the orad plates, Pyrina shows 

 three " Cidaroid " members, with one central tubercle each. These 

 are followed by the fourth plate, which has one tubercle developed on 

 each side of the median one, both being apicad in relation to the 

 central tubercle. In C. sp. there are only two " Cidaroid " plates, 

 and on the fourth plate the first orad additional tubercle appears on 

 the adradial side. C. subrotundus is practically similar, but the third 

 plate has assumed the full characters of the fourth in Pyrina. In 

 C. albogalerus only one " Cidaroid" plate remains. 



Turning to the second column of figures, which show the mid-adoral 

 plates, Pyrina proves to have developed the regular chevron 

 characteristic of Anorthopygus. The median tubercle is distinctly 

 orad in position, and is in tbe same transverse line with the other orad 

 tubercles of the plate. C. sp. is closely similar, but a "triplet" 

 occurs on the adradial side, and the median tubercle is above the 

 average level of the orad series, though still far from central. In 

 C. subrotundus the median tubercle is nearly central, and a super- 

 numerary occurs below it. This supernumerary is always present in 

 ambital plates of both genera, and is strongly reminiscent of the 

 similarly placed tubercle (usually sunken) on some adapical plates 

 of Holectypus depressus and Discoidea cylindrica. C. albogalerus 

 shows the " hour-glass " pattern so characteristic of the species ; the 

 relative simplicity of the tuberculation may be correlated with 

 the small extent of the adoral surface in proportion to the length of 

 the columns. 



In the ambital plates Pyrina shows the true mid-zonal character. 

 The median tubercle is nearly central, there are many supernumeraries, 

 and a considerable proportion of triplets. C. sp. has a group of four, 

 but otherwise the series of Conulus are very similar. It must be 

 remembered that these three ambital plates are situated well below 

 the true mid-zone of their columns. 



Half-way up the adapical surface the tuberculation of Pyrina has 

 recovered its simplicity, and, save for the smallness of the tubercles 

 and the central position of the median one, is very like the 

 corresponding tuberculation of the adoral surface. But C. sp. shows 

 elaboration, passing beyond its ambital quality, while the two other 

 species show little, if any, simplification. 



In the apicad (newly formed) plates a strong likeness is seen in all 

 four cases, but Pyrina introduces extra tubercle series most slowly, 

 and C. albogalerus by far the most rapidly. 



The fundamental agreement of all four species in the ontogenetically 

 young plates (columns 1 and 2) is strongly suggestive of their 

 phyletic relationship, while the introduction of additional tubercles is 

 progressively accelerated. The morphogenetically young plates 

 towards the apex may be regarded as showing prophetic characters 

 (coupled inevitably with those due to immaturity), and from them it 

 may therefore be deduced that Pyrina has reached it full specialization 



