Correspondence — Prof. A. v. Koenen — Mr. A. R. Hunt. 191 



researches of M. Spring and others on the physical and chemical 

 changes produced by the action of high pressures. It seems rather 

 rather late in the day to take this position, but the subject is too 

 wide to be discussed here. The Belgian physicist, too, is well able 

 to defend himself: witness his reply to the American critic cited by 

 General McMahon. Alfred Harkek. 



St. John's College, Cambridge. 



COCCOSTEUS DECIPIENS. 



Sir, — In a very important paper on the structure of Coccosteus 

 decipiens, Ag., Dr. Traquair has recently remarked (Ann. & Mag. Nat. 

 Hist. [6] vol. v. p. 125) that he suspects I have mistaken the lateral 

 margin of the interlateral plate for a pectoral spine in my descrip- 

 tion of Coccosteus, and he feels justified in asserting that, if such a 

 pectoral swimming organ does really exist in C. Bickensis, that 

 species cannot be referred to Coccosteus, in which no such appendage 

 is present. 



In reply, I must repeat that there occurs a hollow, triangular, 

 bony spine, filled with calc spar, quite distinct from the other plates. 

 Apart from this spine, C. Bickensis agrees so well with undoubted 

 species of Coccosteus, that I am inclined to regard Dr. Traquair's 

 statement cited above as not yet beyond question ; and although a 

 similar pectoral organ has not yet been recognized in Scottish 

 specimens, it is quite likely it may still be found. I am all the 

 more confirmed in this opinion since, according to Dr. Traquair, the 

 sclerotic ring appears to exist only in one specimen from Gamrie in 

 the Edinburgh Museum, while it is rather common in my German 

 specimens. The pectoral spine is much more rarely seen in my 

 fossils than the sclerotic ring, and I am thus not astonished that it 

 should hitherto have escaped observation in the Scottish examples of 

 Coccosteus. Finally, I would add that the spine in C. Bickensis 

 attained a length of 55mm. (fig. 12 of my paper on Placoderms), but 

 the end is wanting, the impression of it being retained on the rock. 

 It is therefore not shorter, but much longer than in the restoration 

 of Bracliydeirus inflatus. 



I may add that my specimens are exposed in the Royal Geological 

 Museum here at Gottingen, and may be examined by any one 

 interested in the subject. A. von Koenen. 



Gottingen, March Xlth, 1890. 



TIDAL ACTION. 



Sir, — As tidal action has been called in of late in your pages to 

 assist if possible in solving the riddle of the Triassic sandstones and 

 conglomerates, it may be well to point out one line of evidence 

 which seems to have been overlooked by the supporters of the tidal 

 theory, i.e. the zoological. 



Mr. Mel lard Reade writes as follows in the Philosophical Maga- 

 zine, vol. xxv. p. 342 : — " Although it is on the littoral margins and 

 the shallow seas opening into the oceans that the resistless force of 

 the tides is most obvious," etc., etc. 1 



1 See Mr. Mellard-Eeade's Article in this Number, supra, p. 1 57.— Ed. Geol. Mag. 



