THRINCOCERAS HYATTI. 99 



zone. The interspaces are always wider than the ridges themselves, in some 

 places twice as wide. 



Between the ridges, where the test is well preserved, there are to be seen, with 

 the aid of a magnifier, exceedingly fine longitudinal lines, of which three or four 

 may generally be counted, but in some places only one is present ; they persist 

 throughout the adolescent and adult stages ; I am not able to say positively 

 whether they occur in the young also, but probably they do not, as the closeness of 

 the ridges would scarcely leave room for their development (PI. XXVI, figs. 1 d—f). 



Fine lines of growth cover the whole surface of the test, causing slight nodular 

 excrescences where they cross the ridges. These lines become more distinct in 

 crossing the periphery, where they form a very deep sinus corresponding with 

 that of the aperture (PI. XXVI, fig. 1 a). 



Dimensions. 



Adult specimen from St. Doulagh's, 

 in the Museum of the Royal 

 College of Science, Dublin. 



Diameter of shell . . . .90 mm. 



,, umbilicus (from edge to edge) . . 43 ,, 



,, ,, (from suture to suture) . 35 ,, 



Height of outer whorl (dorso- ventral) . . 28 ,, 



Thickness at umbilical margin . . . 80 ,, 



Width of periphery of outer whorl . . 20 ,, 



Affi7iities. — I point out on page 102 the relationship subsisting between 

 Thrincoceras HiherniGum and the present species. As it was in studying T. Hyatti 

 that I was first led to conclude that the genus Thrincoceras was represented in 

 Ireland by these two species, it will naturally be supposed that T. Hyatti should 

 show a marked resemblance to the American species described by Hyatt, which 

 is, in fact, the case. Of the two species of Thrincoceras described by Hyatt,^ that 

 which he has named Thrincoceras Kentucldense is the most strictly comparable 

 with the present species. Neglecting the superior dimensions of the American 

 form, which has a diameter of 170 mm. against 90 mm. in the largest Irish shell 

 before me, the resemblances between the two are such as easily catch the eye, 

 and, in spite of the very rough figures given in the ' Geology of Texas,' I was at 

 once reminded of them when the fossil, here under description, came under my 

 notice. 



Professor Hyatt's specimens appear to have been poorly preserved, as is 

 indicated in one of the figures he gives of T. Kentucldense, which is a cast showing 



^ " Carboniferous Cephalopoda." Second paper. ' Geological Survey of Texas,' 1893. Eeprinted 

 from Fourth Annual Report, 1892, pp. 430—434. 



