850 Transactions of the Society. 



ceding observers, we are, I think, led to the conclusion that, even 

 in the systematic arrangement of the families of Echinoids, some 

 hints of value are to be obtained from a consideration of the facts 

 of spine-structure ; there is, in fact, a kind of continuity in histo- 

 logical structure which is not always apparent in the grosser 

 details. 



The Cidaridae, with the simplest, that is fenestrated,* type of 

 spine structure, have in some cases, though not in all (e. g. 

 Goniocidaris geranoides, where it is only scattered) a surrounding 

 ring of thicker crust which, as Mackintosh suggests, prevents the 

 softer internal part from being too rapidly worn away ; such a 

 crust is found in the Salenidfle, and at the tip of the spines which 

 in Echinocidaris are found in the neighbourhood of the mouth. 



The fact that it is found in the Salenidae is really in favour of 

 the value of spine structure as an aid to the phylogenetic system- 

 atist, for Salenia does by certain, just as much as it does not by 

 other, characters proclaim its relations to Cidaris. Similarly 

 Echinocidaris stands nearer to the Cidaridae than does Echinus, and, 

 in that portion of the spine which is most liable to be worn down 

 by friction, the interior layer, which is acanthosphenote rather than 

 as in Cidaris fenestrate, tissue, is protected by a crust, (fig. 6.) 



The figures of Salenia ^profundi (now for the first time given, 

 fig. 5) will show how, even in Salenia, there is a tendency to a regular 

 spoke-like arrangement of the inner layers, and a marked reduction 

 in the fenestrate arrangement. Within the limits of the true 

 Cidaridae stages in the extent of the fenestration, and the regularity 

 of the spoke-like intermediate layers are to be observed : when 

 combined with the inquiry into the relations of other structural 

 characters, to which I hope soon to be able to devote myself, they 

 will perhaps be found to be of use in determining the minor 

 questions of the limitations and relations of the genera of which 

 that family is composed. 



That the term Acanthostraca is not synonymous with the 

 name, or the group denoted by it conterminous with that of the 

 Cidaridae, is, after all, only another way of saying that it takes note 

 of only one structural character, and it does not really afford any 

 ground for neglecting a study which is full of interest and 

 instruction. 



The figures of spines, now given for the first time, will be 

 found to explain themselves to those who are acquainted with what 

 has been already done, especially by Prof. Mackintosh. 



* I quite agree with Prof. Mackintosh, who says, "Nor can I agree with 

 Prof. Agassiz that the Cidaridae present us with the most complicated type of 

 spine structure." 



