KONGL. SV. VET. AKADEMIENS IIANDL. 15AND. 19. N:0 6. 117 



EuuiiiplialHs idatm 1854. MoHttis Brit. Fossils Ed. 2, 247. 



1855. PicXET Traitii ilc Pal. Ill, 155, pi. LXll C. 12. 



1862. Mac Coy Synopsis Sil. Foss. Ireland, 13. 



1867. S.^LTER Siluria 3 Ed., 531, pi. 25 f. 4. 



1867. LiNDsTKOM Nomina foss. Gotl., 23. 



1867. QuENSTEDT Handbiicli di;r Pctrci'aklcnkuudi;, 5U6, tal). 45 f. 64. 



1868. BiGSHY Thesaurus Siluricus, 153. 



?1871. B.\iLY Figures of Ciiaraeterislie British Fossils pt. Ill, pi. 21 T. 7 (searceljj. 



1873. Salter Cat. Cambr. Mus., 157. 



1882. QuENSTEDT Petrefakteukunde Deutschlands l:e Abth. 7:e Bd., 327, tab. 197 f. 41. 



^c/ihostoma alatiiiu 1842. D'Archiac & Vern. Trans. Oeol. See. Loud. 2;d Ser. vol. VI, 365. 



StrajiaroUits alatus 1850. D'Orbig.ny Prodr. I, 29. 



Pteurotoiiiaria alata 1858. Fr. Schmidt Esthland, 201. 



Eiiomphalopterus alatus 1876. Feri). Roe.mer Leth. Geoj^n. tab. 14 f. 9 a — 9 b. 



Shell turbiiuite, spire broad and short, consisting of six or seven tubular whorls, 

 circular in section, hg. 29. The slit band is placed above the median line ot the 

 whorl, nearly un a level with the upper edge of the aperture. This large, lamellar 

 slit band, which is built on an exactly homologous plan with that of i'leur. replicata 

 and PI. limata, occupies a belt around the shell, which un the last whorl attains a 

 length across of 12 millni. in the largest specimens, the whole diameter being 50 mm. 

 It is directed at first obliquely upwards, tig. 29, 30, and turns at the exterior edge 

 abruptedly downwards in a curve, which sometimes has the appearance of a hook, f. 80, 

 32. Along its outermost margins this lamina is in some specimens folded in undulations 

 f. 24. If this large slit band be split in two halves along its median line, it is found 

 on each side to consists of two thin strata (f. 27) the exterior one transversally stria- 

 ted on its surface and the interior one quite smooth and transparent. Within these 

 are enclosed the crescentic lamina- and through their development the space between 

 the bordering laminaj seems to be divided into a number of narrow compartments 

 (fig. 27, the enlarged portion). They are Avider apart near their exterior ends and 

 converging towards the whorls. When the slit band is sectioned transversally across 

 these crescents the appearance shown in tig. 30 is produced. The curved outlines of 

 ten obliquely sectioned, crescentic lamellie are seen in the dark shale between the 

 bordering edges of the slit band. These enclosing lamellaj are in direct continuation 

 with the exterior darker stratum of the shell, composed of minute, angular laminie. 

 Within this exterior stratum another of clear crystalline calcite lies, often retaining a 

 nacreous lustre. There are absolutely no communications, nor perforations or tubes 

 between the interior cavity of the whorls and the spaces separating the crescentic lamella; 

 of the slitband as Fekd. Roemek has supposed. The interior cavity is closed as in 

 other Pleurotomariag. Along the exterior edge of the slit band, tig. 31, the ends of 

 the crescents are discerned, especially when a little cut; the thin edges of the baud 

 are usually bi'oken on the older whorls, f. 22. 



The umbilical side of the shell is nearly cup shaped, being hemmed in around 

 the edges by the high, turned up slit band and sunk towards the umbilicus, around 

 which there is an elevation. 



The surface is only transversally striated, the stria; running straight down to 

 the slit band and, chiefly on the umbilical side, directed totoards the aperture and 



