436 SIR J. W. DAWSON ON NAIADITES IN [Aug. 1 894, 



Mr. Salter, however, to whom I sent specimens, regards these shells 

 as belonging to his new genera Anihracomya and Anthracoptera, the 

 former being supposed to be allied to Myadae. 1 More recently 

 Giimbel and Geinitz have described similar shells from Thuringia 

 as belonging to the genera Unio and Anodon, and regard my Naia- 

 dites carbonarius (Anthracoptera carbonaria of Salter) as a Dreissena. 2 

 In the present uncertainty as to their genuine relations I shall retain 

 the name Naiadites for the whole of the species, giving, however, 

 Salter's generic names in brackets." 



In correspondence with Mr. Salter at that time, I had pointed out 

 that these shells were probably freshwater, and objected to his 

 name Anihracomya as expressing an incorrect view of the affinities 

 of the shells that I had sent to him ; assigning the following among 

 other reasons, afterwards published in 1868 in a new edition of 

 ' Acadian Geology ' along with descriptions and figures of the principal 

 species, seven in number : — 



(1) Under the microscope these shells present an internal lamel- 

 lar and subnacreous layer, a thin layer of prismatic shell, and an 

 epidermis, all corresponding to similar structures in the Unionidae. 



(2) The ligament was external; there seem to have been no teeth. 

 The shell was closed (or slightly open) posteriorly, and in some 

 species there are indications of a byssal sinus. The general aspect 

 is in some species that of Unio, in others that of Mytilus. The 

 wrinkling of the epidermis seems to be, for the most part, an effect 

 of pressure. 



(3) I know of no instance of the occurrence of these shells in 

 the marine limestones, or associated with species unquestionably 

 marine. 



(4) The mode of their occurrence precludes the idea that they 

 were burrowers, and favours the supposition that they were attached 

 by a byssus to sunken or floating timber. 3 



(5) The attachment of Spirorbis to the outer surface of many 

 specimens seems to show that they were free in clear waters. 



On these grounds, and being unable from the specimens in my 

 possession to make out evidence of generic distinctness, I continued 

 to use the name Naiadites in preference to adopting the newer 

 names suggested by Mr. Salter. Under this name I have described 

 seven species from the Coal Formation of Nova Scotia, and have now 

 sent specimens of these to Dr. Wheelton Hind for examination and 

 comparison. 



I may add that I do not object to the division of the species into 

 two or more genera, for one of which Salter's name Anthracoptera 

 should be retained. I doubt, however, whether these can be distin- 

 guished by form alone, which in most cases is all that we have 



1 Quart. Journ. Geol. Soc. vol. xix. (1863) p. 80. 



2 Neues Jahrb. 1864, pp.646, 651, and Geol. Mag. 1865, p. 204. 



8 Dr. Hind informs me that a specimen in the British Museum (Nat. Hist.), 

 at South Kensington, has the byssus preserved. [This specimen consists of 

 a piece of fossil wood, round which numerous individuals of Anthracoptera are 

 clustered in several rows, as they would be if attached by a byssus. — W. H.] 



