OCR Text |
Show 416 DR. J. ANDERSON ON A N E W [May 21, on the mesial line of the head and strongly ridged and nodose spines 2 Fig. 1. Fig. 2. E. sclateri, And. E. cethiopicus, Ehr. It has been stated by Dobson, and repeated by others, that the spines of all the known species are marked by longitudinal ridges; but if the spines of E. europceus and its allies be subjected to microscopical examination in cross section, it will be found that they have a perfectly circular outline without any trace of longitudinal ridges, whereas if a spine of E. cethiopicus be treated in the same way the cross section is seen to be thrown into alternate risings and depressions, due to the presence of longitudinal ridges. The cavity of a spine of either of these groups in cross section consists of a number of inwardly projected septa enclosing chambers. In spines with ridges and furrows, the septa always correspond to the latter, i. e. they originate from the inner walls, whereas the ridges are outward bulgings of the chambers defined by the septa. The position of the latter (septa) is generally indicated externally by a dark longitudinal line, due to the greater thickness of the periphery of the spine opposite to a septum. It is the presence of these dark longitudinal lines that has doubtless given rise to the erroneous impression that they are ridges. The 1 I give an enlarged view of the skull of E. sclateri and alongside of it one of E. cethiopicus, to bring out the cranial features of the two groups, |