ACANTHONAUTILUS BISPINOSUS. 121 



angular sinus, through the centre of which there runs longitudinally a slight keel 

 or raised line. The sinus forms a little notch in the margin of the aperture. 



" The greatest diameter of the shell is 100 mm., the greatest width 80 mm., 

 the greatest height 45 ram. ; length of spines, 55 mm. 



" The extension of the last chamber into horn-like processes reminds one of 

 Nautilus Seebachianus, Geinitz (Djas, Bd. i, S. 43, Taf. ix, fig. 7), which has a 

 leaf-like expansion of the body-chamber extending longitudinally and laterally. 

 The likeness of form is shown by the raised line or keel which passes along the 

 centre of the periphery, and is absent in Nautilus Freieslebeni. 



" R. Ludowick, who has seen a specimen of this species in the Geological 

 Museum of the Kazan University, calls it Nautilus Freieslebeni (Dyas, Bd. ii, S. 

 295), and indicates Nijini Tyvesniak, in the vicinity of Kazan, as the place where 

 it is found. This is not correct, as I found it myself, in the year 1859, in the 

 Verchni Tyvesniak, at the mouth of the Tanassalka. It is very often found in the 

 Verchni Tyvesniak, on the Volga, at a little distance between Bourtas and Anto- 

 novka, especially near the village of Krasnovidof." 



From this description it may be gathered that the two species differ in the 

 following particulars : — In Acanthonautilus bispinosus the shell has fewer whorls, 

 and the spines have not the upward curvature which is such a marked feature in 

 the Russian species. Further, the septa are much less numerous in the former 

 species than they are in the latter. There is also a very broad though shallow 

 hyponomic sinus in the aperture of A. bispinosus, while in A. cornutus this structure 

 is scarcely more than a notch. Lastly, the umbilical border in A. cornutus 

 is distinctly angular, with steep sides ; in A. bispinosus, on the other hand, it has 

 a rounded rim. Hence there seems to be amply sufiBcient ground for separating 

 the two species, whose resemblance is restricted to the spines which each 

 possesses. 



Remarks. — It would seem from Golowkinsky's descr'i-ption that Acantho7iautilus 

 cornutus was not an uncommon species — at least in the district where it was 

 found. 



From a list of the fossils of the region given by Golowkinsky in his memoir, 

 it appears that they are typical of the lower Zechstein division of the Djas of 

 Germany. Hence the horizon of Acanthonautilus has been extended vertically by 

 the discovery of the Irish species, which can thus boast of a somewhat greater 

 antiquity than its Russian congener. 



That A. cornutus should have been completely overlooked by palasontologists 

 till within the last few years is, perhaps, not very remarkable, the description 

 being contained in a journal, apparentlj^ not well known out of Russia, printed in 

 Russian, and containing no abstract in German or French by which its valuable 

 contents would be made known to the student unacquainted with that language. 



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