Correspondence. 379 



confidence on tlie subject, having made numerous sections of them 

 in well-determined planes. The specimens submitted for exami' 

 nation appear to have been all very minute examples of their 

 respective kinds, or Prof. Owen would not, I feel sure, have asserted 

 that these " minute fishes " were " of the size of minnows or stickle- 

 backs." Most of these remains have belonged to fishes of no incon- 

 siderable size, some of them, I believe, to the largest fishes of the 

 Coal-measures. The following genera, proposed by Professor Owen, 

 seem to require particular notice : — 



Genus I. Dittodus. — ^This is undoubtedly Diplod^is of Agassiz, 

 and is not a tooth but a dermal spinous tubercle. — I have in my 

 collection vast numbers of such, some detached, others in connection 

 with a thin layer of granular matter, which there can scarcely be 

 any doubt is the skin of some fish. One of these patches in my 

 possession is fifteen inches long and about seven wide. On this the 

 tubercles are comparatively few, and are scattered far apart from each 

 other. But on another patch, measuring fifty-six inches square, they 

 are very numerous, and are crowded together without any apparent 

 order. 



These tiibercles are analogous to the spinous dermal tubercles found 

 on some of the Eays, only in the Eay there is but one spine, while in 

 that under consideration there are usually three, sometimes two, and 

 rarely only one ; when three are present, one is always much smaller 

 than the other two, and rises from the common bony base behind 

 them and opposite to the space between them. So that in making a 

 section to exhibit the two principal spines it is almost impossible to 

 preserve the small posterior one-; consequently it happens that only 

 the two large spines are seen united at the base — "the two crowns" 

 as Professor Owen expresses it, being "organically connected to- 

 gether, like the Siamese-twins." 



Diplodus varies verj'- much in size, being occasionally quite micro- 

 scopic, and not unfrequently measuring nearly three-quarters of an 

 inch in length. It also varies considerably in character, the spines, 

 or ' crowns ' being frequently long, nearly parallel, and compara- 

 tively slender; occasionally they are found diverging, short, stiff, 

 and much bent ; numerous specimens occur having characters inter- 

 mediate to the two extreme forms ; there can, therefore, be no doubt 

 that Dittodus parallelus and D. divergens are mere varieties, the one 

 of the other. 



Grenus II. Mitrodus. — This, I believe, will also prove to be a der- 

 mal tubercle. There are occasionally found in the Low Main shale 

 thin layers or patches almost entirely composed of minute compressed 

 bodies, having rising up from their upper or free margin from two to 

 seven conical denticles, which answer very correctly to the account 

 given of this form. I possess such a patch measuring 20 inches long 

 and 13 inches wide. It is spread over with vast multitudes of these 

 tubercular bodies, which are crowded together in a confused manner. 

 Detached specimens of the tubercles also occur. 



These tuberculated patches are usually associated with the spines 

 and other remains of Gyracanthus, and are most probably the skin 



