Echinoid Tests, Diademoida. 
113 
trouble is that this inequality appears to be quite unrelated to the differences 
of tuberculation. 
In criticising the diagnoses offered by Messrs. Lambert and Valette, 
I do not deny the possibility of sorting the species into two groups according 
to such characters as size of secondary tubercle-series or width of inter- 
ambulacrals. Evidently any series of objects could be divided by so arbitrary 
a measure as the ratio of height to width, just as by a sieve. But no evidence 
has yet been adduced to show that these divisions correspond to genetic 
groups, and I cannot find that any such correspondence exists. This does not 
prevent me from believing that there were at least two lines of descent, 
represented respectively by Diademopsis serialis and Hemipedina Etheridgei, 
nor do I suggest that there is any particular difficulty in assigning the pre- 
Bathonian species to one or other of those series. The difficulty is that the 
Hemipedina series may have produced, and probably did produce, subsequent 
species which the criteria hitherto suggested do not enable us to distinguish 
from Diademopsis. 
The practical outcome of this discussion is the retention of Hemipedina 
(sensu lato), with a diagnosis that shall permit the inclusion of species with 
any number of interambulacral tubercle-series and with any ratio between the 
width and height of the ambital interambulacrals. At the same time it is 
convenient to admit the subgenera Hemipedina s. str. and Diademopsis , for 
those earlier paucituberculate and multituberculate species respectively, as to 
the genetic independence of which there is little room for doubt. 
Of the genera that find place in this discussion there is now left only 
Mesodiadema. This, with the species referred to it, will be dealt with indepen- 
dently in the following section ; but it may here be pointed out that the interam¬ 
bulacral plates of Mesodiadema differ from those of Diademopsis in bearing only 
one primary tubercle on each, and from those of Hemipedina in having that 
tubercle relatively small and unaccompanied by any trace of secondary tubercles. 
Its ambulacral plates also differ from those of the genera proviously discussed in 
being all majors with strictly uniserial pore-pairs throughout. 
Mesodiadema. 
1889. Mesodiadema M. Neumayr «Stämme des Thierreiches» p. 372. 
1900. Mesodiadema Neumayr, J. Lambert, Bull. Soc. Sei. Yonne, L1II (1) p. 31. 
In addition to Quenstedt, 1875, and Duncan, 1889, see also: 
1882. P. de Loriol, Mem. Soc. phys. Geneve, XXVIII, No. 3, p. 8. 
1904. Y. Delage & E. HfiROUARD, «Traite de Zool. concrete», III, p. 233. 
1905. C. Airaghi, Atti Soc. Ital. Sei. nat. [= Atti Mus. Milano], XLIV, p. 4. 
Diagnosis. — A Diadematoid with ambulacrals never compound or tuberculate, 
but bearing miliaries; with unigeminal pores; with each interambulacral bearing 
a perforate, noncrenelate tubercle. 
Genotype: Hemipedina Marconissae Desor ex Meneghini MS., from the 
Toarcian, zone of Terebratula Aspasia, Tuscany. 
Resultate der wissenscliaftl. Erforschung des Balatonsees. I. Bd. 1. T. Pal. Anh. 
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