SOLEXOCHEILUS DORSALIS. ]27 



last. At the edge of tlie umbilicus, w^liere the lower margin of the aperture meets 

 the lateral margins, there is a distinct swelling, a characteristic feature seen in all 

 individuals of the species in which the aperture is preserved at this point (PI. 

 XXXIII). With the complete form of the apertiire and its hyponomic sinus I am 

 unacquainted, as I have not met with any specimens in which it is preserved ; nor 

 are the lines of growth sufficiently distinct to indicate its nature in the specimens 

 known to me. 



The septa, as indicated by their sutures, are wide apart. In a large specimen 

 from Rathkeale, near Limerick (PI. XXXV,' fig. 1), the diameter of which is 

 approximately 230 mm., the distances between the sutm^es in the last six chambers, 

 measured along the median line of the periphery (reading them towards the body- 

 chamber), are as follows: — 28 mm., 28'5 mm., 29 mm., 34 mm., 32 mm., 35 mm.^ 



The siphuncle has a diameter near the body-chamber of al)out 4 mm. ; it lies 

 immediately under the test, so that the removal of the latter exposes it. The test 

 is marked only by very faint lines of growth. 



Affinities. — The most nearly related species is Solenoeheilus ecolutus, Hyatt ^ 

 (= Nautilus dorsalis, de Kon.,* not Phillips), but the whorls in this species are only 

 just in contact instead of overlapping considerably as is the case with S. dorsalis, 

 Phill. Hence the renaming of de Koninck's species by Hyatt. 



From S. latiseptntus, de Kon.,^ the present species differs in its much narrower 

 periphery, more rapid rate of tapering, and overlapping whorls, as well as in its 

 relatively closer septation. 



Remarls. — The Irish specimens of this species agree v/ell with Phillips's type 

 (form a, from Bolland, Yorkshire), which I had the oppoitunity, through the 

 kindness of Dr. Henry Woodward, F.R.S., of re-examining at the British 

 ]\Iuseum in the summer of 1900. They have the characteristic narrowly rounded 

 periphery possessed by the type specimen. The sutures are not seen in the latter, 

 which has, however, part of the body-chamber intact. The test is badly pre- 

 served, its surface having been entirely destroyed by a crystalline deposit. 



S. dorsalis is ■widel)^ distributed in Ireland, where it attains a very consider- 

 al)le size. The largest I have met Avith is now in the British Museum ; it was 

 found in the quarries at Clane. I give its dimensions, kindly supplied to me 



' This plate, having been added after the publication of Part III of the present Monograph, is 

 not enumerated at the head of the description of this species on page 126. 



^ These measurements will not be found to agree precisely with the drawing, owing to the position 

 of the lines in the latter being naturally affected by the curv'ed surface of the shell. 



' " Carboniferous Cephalopods." Second paper. ' Geological Survey of Texas, Fourth Annual 

 Report,' 1892, p. 460. 



* "Faune Calc. Garb. Belg.," 1878 (-Ann. Mus. Roy. d'Hist. Nat. Belg.,' torn, ii), p(. 1, p. Ill, 

 pi. xviii, figs. 1 — 3. 



» Ibid., p. 110, pi. xxii, figs. 1—3. Also ' Cat. Foss. Ceph. Brit. Mus.,' 1891, pt. 2, p. 171, fig. 29. 



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