Studies on the Echinoidea. No. XI, 431 
and Globator), already described, may be referable to the genus 
here introduced ; but more details of morphology than are often 
supplied in systematic diagnoses and figures are needed before order 
can come out of the chaotic series of Cretaceous Echinonéide. 
I am convinced that the shape of the test is a feature of practically 
no importance in specific determination in the group; while even 
the position of the periproct is reliable only when specimens of the 
same size from accurately determined horizons are compared. The 
apical system shows a remarkable constancy of structure throughout 
the family, and so is useless for detailed taxonomy. The obliquity 
or symmetry of the peristome is another morphological character 
often invoked as a generic criterion; but I have yet to find a 
specimen of Conulus without some degree of asymmetry in that 
aperture. There remain but two external characters for comparison, 
the surface-ornament and ambulacral structure. Both of these 
demand good preservation for detection, and patient study for 
appreciation ; the latter attribute has too often been lacking in 
descriptions (and particularly in drawings) purporting to represent 
these details. 
In the note introductory to this series of papers (Grou. Mag., 1917, 
p- 161), I laid emphasis on the essential conservatism of the 
Holectypoida, and the systematic difficulties thereby engendered. 
In no family of the order is this static quality of essentials more 
manifest than in the Echinonéide. No investigation that does 
not take into account stratigraphical and morphological features 
more delicate than suffice in some other groups can have much hope 
of success. It is with delight that I learn that my friend Dr. A. W. 
Rowe is following up his classic study of Micraster with a similar 
treatment of Conulus. In that way alone can the continuous or 
gently oscillating course of evolution of pelagic Echinoids be traced. 
But for the relatively littoral types, among which Pyrina must be 
classed, disjointed fragments of history are alone available; so 
that every detail needs accurate description, lest an inconspicuous 
feature that later proves a reliable guide should be overlooked. 
Thus, and thus only, may it be possible for some future worker to 
ascertain the distinctive characters of Conulus and Pyrina, which 
Lambert places in separate orders, while I cannot undertake to 
point out even generic differences that apply to all of the species 
therein included. 
In respect of the two characters above indicated as possessing 
taxonomic value, it seems possible to find a rough distinction between 
Conulus and Pyrina in the following comparison. The inter- 
ambulacral tubercles of Conulus are relatively small, with shallow 
areole, adapically ; and larger, with deep areole, on the ambitus 
and adorally. Those of Pyrina are less distinctly differentiated, 
the tubercles of both surfaces being fairly large and the areolx 
narrow but deep throughout. In both genera a chevron pattern is 
developed by the tubercles of each interambulacral plate, but in 
