92 G. LINDSTROM, UN THE SILURIAN GASTROPODA AND PTEROFUDA OF GOTLAND. 



Besides the above eiiuiuei'ated genera, wliieli have been exelusively, or abnost 

 so, instituted foi" the reception of species ot Pleiirotouiaria, oilier genera, which have 

 nothing in coiiiuion vvitli that genus, shelter a few true Pleurotouiariie, but have not 

 been phiced in the bst of synonyms, as their authors did not intend them for I'leuro- 

 tomaria;. So, for inst. Pleurotomaria eUiptica His. has been by De Koninck included 

 in the Trochoid genus Fleniingia, established by him, but such instances are mentioned 

 further down in the descriptions and synonyms of the species. 



It must be conceded tliat the limits between this genus and Murchisonia are very 

 difMcult to draw, a wide field being given to arbitrary individual opinions through tiie 

 vagueness attributed to the chief character of Murchisonia, viz. the length of its spire. 

 It may, nevertheless be practically useful to distinguish such Pleurotomariiu, which have 

 a long, slender spire of more than six, beadlike whorls ( — Hormotoma Salter) as 

 Murchisonia. In them the nacreous nature of the shell is not so evident as in Pleuro- 

 totnaria proper. The nature of the Plcurotomarian shell as nacreous has been much 

 contested, but it admits of no doubt through several Gotlandic specimens, which still 

 retain the pearly coating interiorly. 



The character of the slit in the Silurian species is rather deviating from what 

 commonly is represented as its form in secondary and more recent formations. The 

 lips are in these straight cut and the borders of the slit parallel, but in the Silurian 

 ones the slit forms an acute angle and the borders are diverging. In some the slit is 

 linear, as in Peurotom. a-quilatera. As to the slit band, one of the most characteri- 

 stic features in this genus, it deserves more than a passing attention. It is in almost 

 all instances hemmed in by at least two, parallel, elevated lines, one to each side, al- 

 ways distinct. The band itself is of a varying breadth, linear as in Pleur. gradata, pi. 

 VII, tig. "26, or large, relatively, as in Pleurotomaria exquisita, pi. XI lig. 3. The usual 

 ornamentation consists in crescent-formed lines of growth, pi. IX, ligs. 2, 4, 6, 15 etc., 

 with tlie concavity towards the aperture. These crescents are sometimes crossed by 

 longitudinal lines as in PI- clanstrata pi. VII f. 31, 34, PL glandiformis, PI. biformis, 

 pi. VII f. 42 or by a longitudinal ridge as in a whole group of Pleurotomaria with 

 PI. bicincta pi. VIII figs. 19, 21, 22, 23 as their type. In PI. limata, i)l. X ligs. 14— 

 l(j, the lamellae of growth have a most extraordinary appearance being thin, prominent, 

 bent forward, imbricated and in their midst divided through an ovate indenture. The 

 bordering lines are either very low, of middling size or developed to an enormous 

 length as lamellae. There can be discerned certain gradations, by which the extreuje 

 development in PI. alata is connected with the others. In PI. claustrata, pi. VII 

 f. 32, both the bordei'ing lines project so much as to transform the slit band into a 

 deep groove around the shell. In PL limata, pi. X lig. 8, 14, they stand forth and 

 form a sharj) keel around the shell. A magnified section of it is represented in pi. X 

 fig. 14, being included in soft limestone, whereby the most delicate parts are preserved. 

 The cusps inside are sections of the crcscentic lamelhe of growth being cut obliquely. 

 In 1)1. X fig. 17 a cloven, aliform slit band is figured. The moieties of two lamelhe are 

 seen in their length from the broad, scooped out basis to the pointed apex. In pi. 

 X figs. 27, 31, 37, details are given of the slit band of PL alata showing it, in fig. 27 



