416 



DE. J. ATTOERSON OX A NEW 



[May 21, 



on the mesial line of the head and strongly ridged and nodose 



spines 



Kg. 1. 



Fig. 2. 



E. sclateri. And. 



E, mtkiopiciis, 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. evrojjceiis and its allies be subjected to 

 microscopical examination in cross section, it will be foimd that 

 they have a perfectly circular outline without any trace of longi- 

 tudinal ridges, whereas if a spine of B. cetJiiojjicus 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 



' I give an enlarged view of the skull of E. scJateri and alongside of it one 

 of E. aihiojjicus, to bring out the cranial features of the two groups. 



