66 G. LINDSTROM, ON THE SILURIAN GAS rK( )1'()I)A AND PTEROFdIJA OF GOTLAND. 



(pi. II fig. 52), there are at irregular intervals longitudinal, revolving grooves one or 

 even as many as three and the surface with its lines of growth assumes a scaly or 

 imbricate appearance. In one variety from the lowest shale beds of Wisby there is 

 regularly a groove and an elevated ridge around the umbilicus. 



In another form of the aperture there are large tonguelike lobes (pi. II fig. 41 

 & 42), giving not only the aperture, but also the shell an irregular shape. 



On the inner side there are, as can best be seen through the casts (pi. II figs. 

 44 & 45), large transverse ridges or weals especially in the last volution and which 

 are seldom seen on the outside. Although I have carefully searched for muscular scars 

 on the inside of many well preserved specimens, I have never succeeded in finding 

 such. On the inside of the body whorl a pocket lens reveals a peculiarly streaked 

 surface of bifurcated and sinuous or branching lines with jagged borders in faint relievo 

 on a dark surface. PI. Ill fig. 23. A horizontal, thin section, made deeper below this sur- 

 face presents the mottled appearance figured on plate III figs. 25 — 26. Vertical sections of 

 the shell, magnified through the microscope are seen in plate III figs. 19 — 22. Of these 

 tig. 22 shows three distinct strata, the outermost very thin, the median one composed 

 of transparent prisms, the innermost, which is the thickest, is darkened through inter- 

 spersed, small, black grains, which probably have been added during the process of 

 fossilization. In other sections this stratum is composed, figs. 19 — 20, of angular lines 

 of growth, resembling those which are seen in PI. prototypum. 



Dimensions of the specimen figured pi. II fig. 32 — 33. Height 21 mm., breadth 24 

 inilliin. Spec. tigs. 39 — 40 height 30 millim., breadth 32 raillim. Specim. figs. 34 — 35 

 height 15 millim., breadth 15 millim., height of spire 5 raillim. Largest specimen, from 

 Lindeklint, 42 mill, in height, 41 mm. in breadth. 



This, the most common of all Silurian Gastropoda of Gotland, has been found 

 almost everywhere, and in all strata between Hoburg and the northernmost point of 

 FarO. The largest specimens are as a rule found in the higher strata, as in the red 

 limestone of Sandarfve kulle and Linde. The old truth that species which are widely 

 spread and very rich in individuals, also are highly variable and as a rule even have 

 a long duration in geological time, holds in the fullest sense good with this species. 

 The first impression one gets when glancing at the bewildering dissimilarity of forms 

 delineated as Platyceras cornutum on the plates II and III is that we have to deal with 

 a great many different not only genera but also species, and the above list of synonyms 

 shows indeed how the opinions of the authors run in many different directions. But 

 by patiently comparing a large number of well preserved specimens from the same 

 locality, the conviction forces itself upon the mind, however reluctant it may be, that 

 they all belong to the same species. Variable as many of the Silurian Gastropoda are, 

 this is the most variable of them all. It is difficult to decide whether this species is 

 one of those plastic groups which truly may be called the main source of several diffe- 

 rent new specific forms or if it ])e one of those mocking species which easily assume the 

 shape and features of others, often not at all related. 



The most common of the Gotland forms is that which resembles SowEnr!Y'.s Ne- 

 rita haliotis. It seldom attains so large a size as the English specimens, but it enti- 



