344 Br. H. B. Boll— On Fossil Sponges. 



an inherent difference in the nature of the animal, however much 

 they may resemble one another in external character. It will be 

 desirable, therefore, to consider the value of these organs for a 

 moment before proceeding further, and in so doing it will be well to 

 turn to the living sponges for aid. 



1. Form. — As already observed, form, taken alone, is of little 

 value even as a specific character, for whether it be cup-shaped, 

 tubular, or polymorphous, it is not distinctive, inasmuch as it is 

 common to sponges widely differing in all other characters, and 

 indeed, as remarked by Dr. Bowerbank, trusting too implicitly to 

 outward configuration has led to the placing of spicular and fibrous 

 sponges side by side in the same genus. Moreover, the form varies 

 greatly at different periods in the growth of the same individual ; 

 and even in the cup-shaped sponges, commonly the most constant as 

 regards this character, there is a wide difference in the figure of the 

 old and young individuals o£ the same species. Hence it is that we 

 know so little of the young condition of many of the fossil sponges, 

 which are not recognized as such, but are regarded, for the most part, 

 as distinct species. 



A circumstance which illustrates how the form of the sponge is 

 liable to be governed by accident is mentioned by Dr. Bowerbank in 

 speaking of their reproduction, and is so suggestive that I quote his 

 own words : "On a fragment of a bivalve-shell 20 or 30 sponge-gem- 

 mules had located themselves, the largest of which did not exceed 

 ^-i-^ of an inch in diameter, and their distance apart is about equal to 

 their diameter. In their present state," says Dr. Bowerbank, " it is 

 evident that they are separate developments ; and it is equally 

 evident that a slightly further amount of extension would have 

 caused them to merge into one comparatively large flat surface of 

 sponge. We see, by this instance, that a sponge is not always de- 

 veloped from a single ovum or gemmule, but, on the contrary, that 

 many ova or gemmules are often concerned in the production of 

 one large individual ; and this fact may probably account for the 

 comparatively very few small sponges that are found." ^ Thus the 

 form of the sponge may be modified, in some instances, by the 

 number of gemmules or ova that may happen to be grouped together, 

 for it is well known that sponges of the same species readily unite 

 when in contact. 



It is necessary to bear these facts in view, for in most of the 

 higher groups of life, whether living or extinct, variation in form is 

 restricted to within very narrow limits, and therefore it is one of 

 the most important characters we possess in the determination of 

 the species. Nevertheless, even in the sponges it is not without a 

 value, for in certain fossil genera the more matured individuals 

 appear to be tolerably constant in this respect, as for instance the 

 Ventriculites, Ischadites, Guettardia, etc., but at the same time the 

 young condition of these genera are unknown to us, or if so have 

 been regarded probably as altogether distinct ; the youngest indi- 



1 I. c, p. 146. 



