186 G. LINDSTROM, ON THE SILURIAN GASTROPODA AND PTEROPODA OF GOTLAND. 



any scar of" the attachment. The whorls are in most specimens not apperceptible on 

 the outside, but through sections it has been made out that there with certainty are 

 six. They are almost smootli, not separated by any regular suture and are finely and 

 transversally striated by exceedingly narrow lines. These, which run in an oblique 

 direction, are crossed by more conspicuous, though minute lines and parallel with these 

 or with the umbilical border there are irregular ridges or constrictions or even edges 

 of the lines of growth. The affixed apex has often been strengthened by radiciform 

 offshoots, which give the shell a strange, nearly corallian aspect, and this is also the 

 case when two Autodeti have grown close together and, as it were, clasp each other 

 with their roots. In the youngest specimens the ornamentation, pi. XXI f. 58, is 

 better preserved and we see that the oblique, transverse lines are by far larger than 

 the delicate, longitudinal ones. The umbilical surface is much more distinctly circum- 

 scribed in this shell than in any other, excepting, perhaps, such forms of Troehi as T. 

 cavus and T. profundus. Its borders are very thin and seldom entire, but broken and 

 fragmentary. Inside the surface is moderately concave, elevated in the centre around 

 the axis in a faint convexity. Around this or as far as the thin margins reach, a 

 wreath of small bladders, of much unequal size in different specimens, as seen by figs. 

 18 and 22 pi. I, is stretching. These and the whole central surface are covered by 

 minute, wartlike prickles, amongst which curved stria? radiate from the axis to the 

 periphery. There is no umbilicus and the axis is solid. The aperture is a narrow, 

 transverse slit with the upper lip laminar and ending in a blunt, broad, triangular 

 spine, which is bent a little upwards near the centre of the umbilical side. The inner 

 lip is thick and is distinctly separated from the surface. Around the inferior side of 

 the aperture the shell has, as in the recent Phoridte, a covering of a thin, glossy stra- 

 tum of porcellaneous shell matter. 



A longitudinal section, pi. I fig. 23, through the axis reveals five or six whorls 

 of elliptical shape and nearly similar to those seen in other regular shells and not at 

 all analogous to the spiral laminaj in Calyptra3a and Trochita. But there is a charac- 

 teristic feature in it, worthy of attention, and consisting in an accumulation of small 

 bladder like cells along the inferior corner near the wall of the whorls. These blad- 

 ders, in some cases amounting to as many as six above each other, are thus comprised 

 in an angle between the roof of a lower whorl and the floor of the next. They form 

 the circle along the inside of the umbilical border, described above, and may there be 

 seen in their original shape when uncovered. In pi. I fig. 19, rows of these bladders 

 are seen in a corroded specimen, indicating the wanting lines of the suture. These 

 bladders are then evidently formed on the top of the whorls along the thin rim of 

 the umbilical surface and are again covered during the growth of the shell by the 

 new whorls. By pi. I fig. 18 it can be found that they originate, as could be ex- 

 pected, near the outer corner of the upper apertural lip and thus had been formed 

 along with the aperture. As to the nature and homologies of these bladders, I can 

 for the present only compare them with the interior partitions through transverse la- 

 mina? in several shells as in Triton corrugatus Lam. ^). But there is the essential dif- 

 ') Woodward Manual of Shells, ed. 1, 100. 



