APUELECERAS MUTAUILE. 67 



most prominent near the suture of the shell, and become gradually finer from this 

 point to the edge of the cavity ; they are continued iipon the dorsal area, but here 

 they are again much finer, and become obsolete at a short distance from the 

 initial point, at a place where the shell measures about 4 mm. across the sides. 

 The ridges are crossed by very fine and close-set lines of growth, making a 

 delicate and beautiful cancellated sculpture. This sculpturing is not continued 

 beyond the innermost volution, and scarcely lasts for a complete whorl, the 

 test becoming abruptly smooth or marked only by feeble lines of growth, 

 except upon the periphery, where these lines are regular and very crowded, 

 making a sinus which bends sharply back in conformity with the edge of the 

 aperture. 



Dimensions. 



Spt'ciinon (roin Clane in tlie Museimi 

 of Science and Art, Dublin. 



Diameter of shelP .... 111mm. 



,, umljilicus (from edge to edge) . . 60 ,, 



„ „ (suture to suture) . . 51 ,, 



Height of outer whorl (dorso-ventral) . . 85 ,, 



Width of periphery of outer whorl . . . 12 ,, 



Thickness of shell at umbilical edge . . . 25 ,, 



Depth of umbilical wall . . . . 5 ,, 



The rapid increase in the growth of the shell is very strikingly shown by 

 measuring the whorls transversely, from the inner to the outer whorl, when the 

 figures are as follows: 5 mm., 14 mm., 35 mm. 



A very large specimen in the Museum of Science and Art, Dublin, yields the 

 following measurements : diameter of the shell, 123 mm. ; that of the umbilicus, 

 68 mm. ; height of outer whorl, 38 mm. 



Affinities. — The most nearly related species is Apheleceras discoideum, de Kon., 

 sp.,' but this is easily distinguished from the present species by its much more 

 compressed form, and by the spiral ridges which extend to the beginning of the 

 last whorl. Another related form is Aplielceras difficile,^ de Kon., sp., in which 

 the whorls are much wider transversely ; they also overlap the preceding ones to 

 a much greater degree than is the case in A. mnfabile, and the central vacuity is 

 much smaller and the septa more numerous than in the latter. A. trochlea, 

 M'Coy,* also bears some relationship to A. mutahile, but it is distinguished by its 



1 The specimen measured has been rendered somewhat elliptical, apparently tlirouyli rock 

 pressure, wiiich has operated in a direction contrary to that of the spiral axis of tlie whorls. 

 - ' Faune Calc. Carb. Belg.,' i)t. 1, p. 133, pi. xxv, fig. 3. 

 ■5 Ibid., p. 118, pi. xxvi, fig. 5. 

 * 'Syuop. Carb. Foss. Ireland,' 1844, p. 19, pi. iii, fig. 4. 



