92 G. LINDSTRÖM, ON TIIE SILUKIAN GASTU01'0DA AND PTEKOPODA OF GOTLAND. 



Bcsides tlie above eiiuiULTutccl genera, whicli liave bceii exchiöively, or aluiost 

 so, instituted for the reception of speciets of Pleurotoinaria, other genera, whicli iiave 

 notliing in conunon with that genus, shelter a few true Plcurotoniariiv, but liave not 

 beeii placed in the list of synonyms, as their authors did not inteiid theni for 1'leuro- 

 tomarite. So, for inst. Pleurotoinaria elliptica His. lias been by De Koninck ineluded 

 in the Trochoid genus Fleuiingia, established by him, but sucli instances are mentioned 

 further down in the descriptions and synonyms of the species. 



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

 difficult to draw, a wide iield being given to arbitrary individual opinions through the 

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

 It may, nevertheless be praetically useful to distinguish such Pleurotomariie, wliich have 

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

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

 tomaria proper. The nature of the Pleurotomarian shell as nacreous has been much 

 contested, but it adniits of no doubt through several Gotlandic speciniens, wliich 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 jjarallel, 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. asquilatera. 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 herarned in by at least tvvo, parallel, elevated lines, one to each side, al- 

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

 VII, fig. 26, or large, relatively, as in Pleurotomaria exquisita, pl. XI iig. 3. The usual 

 ornanientation consists in crescent-formed lines of growth, pl. IX, tigs. 2, 4, 6, 15 etc, 

 with the concavity towards the aperture. These crescents are soraetimes crossed by 

 longitudinal lines as in Pl- claustrata pl. VII f. 31, 34, Pl. glandiformis, Pl. biformis, 

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

 Pl. bicincta pl. VIII figs. 19, 21, 22, 23 as their type. In Pl. limata, pl. X tigs. 14— 

 16, the lamellaa 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 enornious 

 length as lamella;. There can be discerned certain gradations, by which the extreme 

 development in Pl. alata is connected with the others. In Pl. claustrata, pl. VII 

 f. 32, both the bordering lines project so miich as to transform the slit band into a 

 deep groove around the shell. In Pl. limata, pl. X fig. 8, 14, they stånd forth and 

 form a sharp keel around the shell. A magnified section of it is represented in pl. X 

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

 The cusps inside are sections of the crescentic lamelUii of growth being cut obliquely. 

 In pl. X fig. 17 a cloven, aliform slit band is figured. The moieties of two lamella3 are 

 seen in their length froiu the broad, scooped out basis to the pointed apex. In pl. 

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



