Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/88

58 Considering that the abortion of the anterior poriferous zone of the antero-lateral ambulacra has been so often figured in Hemipatagus, it is remarkable that a more strict search after an internal fasciole, which could alone produce that peculiarity, has not been previously successful. The best Mordialloc specimens indicated it perfectly; and many old and worn specimens give indications of its presence now that one knows where to look for it. The subanal linear fasciole is situated just where the posterior and truncated (actinal) part of the test touches the surface on which the test rests; hence it is readily worn off, after the spines have been removed, by the ordinary attrition to which fossils are subject.

Some confusion has arisen in the nomenclature and in the specific distinction of the Australian Hemipatagi (now Loveniæ) already known, owing to my communication to the Ann. and Mag. Nat. Hist., ser. 3, vol. xiv. Sept. 1864, having been overlooked by Laube, whose admirable essay on the Fossil Echinida of the Murray cliffs was read before the Vienna Academy in 1869. He redescribed the same species which I had called Hemipatagus Forbesi, Woods and Duncan. In 1875 Mr. R. Etheridge, jun., described in the Quart. Journ. Geol. Soc. vol. xxxi. No. 123, p. 445, a species which he has called H. Woodsi, and in order to define it from H. Forbesi had the latter figured in outline giving a side view. This figure does not correspond with mine in Ann. and Mag. Nat. Hist. ser. 3, vol. xiv. plate vi. fig. 3; nor does it with Laube's. Again, my figure and Laube's are not exactly alike; for a certain amount of variation must be conceded to all the species of this genus; and the figure in the 'Sitzungsberichte ' has a more truncated and less depressed anterior front test than mine. Mr. Etheridge's figure is shorter and more truncated still, and approximates to his new species H. Woodsi, so as in fact to restrict the specific diagnosis to the greater or less number of large tubercles with sunken scrobicules in the two lateral interambulacral spaces on either side.

Laube evidently had better specimens than those from which I described H. Forbesi. It appears, after an examination of a numerous series (35 to 40) of Hemipatagi from the Australian Tertiaries, that there are groups embraced under the species H. Woodsi and H. Forbesi, Etheridge; but they are connected by intermediate forms. There are H. Woodsi, with flat and with truncate ends, with oval or transversely elongated periprocts, with numerous tubercles in the lateral interambulacra or with few. The larger H. Forbesi resemble the H. Woodsi in shape, and have variable numbers of tubercles; and the number even differs on the opposite sides of the same individual.

The depth of the anterior furrow, the distinctness of the subanal fasciole, and the width of the ornamentation included by the internal fasciole vary in specimens possessing perfect resemblances in other structures. The number of pairs of pores in the ambulacra is, in the majority of the specimens, as follows (taking 15 as good types):—