1887.] NATURAL SCIENCES OF PHILADELPHIA. 18.3 



The following brief summaries of specific points may serve as a 

 guide to the intending student, enabling him at a glance to select 

 the species whi^h his specimen most nearly resembles, without hav- 

 ing to read many pages of inapposite matter. (It must be remarked 

 in explanation of the omission of some names, that the verbal de- 

 scriptions of some of the older species, as copied from their authors, 

 fail to give diferential points that can be made use of in this con- 

 nection. In the treatment of my own discoveries or those of others 

 that have come to me for examination, I have desired to be thor- 

 oughly conservative, grouping those together that have an un- 

 doubted relation to one another and not creating either species or 

 varieties unless they appear necessary to aid in study of the subject. 

 I hesitate however to drop species, the type specimens of which I 

 have never seen ; although it is probable some of them might be- 

 come synonyms to advantage.) 



KEY TO THE SPECIES OF THE GENUS SPONGILI.A. 



(a) Sponge branching. 



1. Slender, cylindrical, waving branches ; dermal spicules minute 

 smooth acerates; gemmules few, sponge evergreen. (PL VIII 

 (fig. vi. S. aspinosa. 



2. Branches generally tapering, rigid ; less frequently cylindrical 

 and flaccid ; skeleton spicules smooth ; dermals pointed, spined 

 acerates ; gemmules after maturity numerous ; with or without 

 granular crust ; spicules cylindrical, curved, spined. (PI. VII, 

 figs, i-vi.) S. lacustrin. 



3. Branches small ; crust of gemmules thin, spicules smooth. 



8. r he nana 

 (b) Sub-branched. 



4. Spines of dermal spicules longest at the centre; gemmule spicule 

 round-ended, covered with recurved spines. 



S. alba. 



5. Short compressed branches; gemmule spicules at various angles. 



S. cerebellata. 



(c) Sponge without branches. 



6. Gemmulse with thick crust of polyhedral cells arranged per- 

 pendicularly; spicules of gemmulae smooth. S. carteri. 



7. Crust as in the last species ; a layer of minute spined acerates 

 intervened between it and the chitinous body, besides that which 

 is exterior to it. S. nitens. 



