56 G. LINDSTRÖM, ON TIIE SILUKIAN GASTROPODA AND PTERorODA OF GOTLAND. 



to be composed of prismatic cells. Near the apex this outer stratum is entirely pris- 

 matic. It is generally of a whitish hue, contrasting with the interiör one, which is 

 yellow. The låter is very thin, consisting of minute lainella3 of an apparently prismatic 

 structure. A singularity in structure is that figured on pl. III f. 4, a longitudinal 

 section, showing the extreinely thin lamella3 perforatcd, as it were, by narrow tubes. 

 These cannot, as the following, be extraneous, låter formed organisms, but must have 

 originated at the same time as the laminas of the shell, as it is clearly seen that these 

 lamina3 are bent exactly where the tubes are situated. 



Almost the whole exteriör stratum is closely perforated by what seems to me 

 to be a different, parasitic organism. When seen with a pocket lens of sufRcient power 

 the surface is pitted by a number of irregularly placed, creara-coloured points (pl. III 

 f. 5) which are soraewhat elevated above the surface. In a thin, vertical section these 

 white döts are seen to prolong downwards through the lamella? of the shell as tubes 

 which generally branch or anastomose so as to form three or four or even raore branch- 

 lets (pl. III f. 1), all filled with the same uniform cream-coloured matter. In a trans- 

 verse section they are also remarked to subdivide and to form openings of stellulate 

 appearance (pl. III f. 2). Similar fossil and recent organisms have been found already 

 long before. Duncan ') has described some small microscopic organisms, which pene- 

 trate the corals of the Devonian and Silurian times. Kölliker ^) has also more in full 

 described a great number of such minute, parasitic forms from several invertebrate 

 animals. Thus he mentions nine species of Gastropoda with perforated shells and he 

 makes the same conclusion as Wedl before him, that this is due to fungous growth 

 or to the mycelium of microscopic fangi. The white radiating tubuli in the shell- 

 substance of Trybl. reticulatum make the impression of haviiig been at first open and 

 then to have been filled with liinestone of another kind. In the nearly related species 

 from Esthonia no such tubes are visible. 



Length 40 millim., breadth 25 mill., height from margin of aperture 8 millim. 



Found in the northern strata of Gotland, in Fårö at Länsa, Lutterhorn, in the 

 limestone of Wialmsudd near Fårösund, Svarfvare huk, in the canal at Westöös in 

 Hall and in the uppermost limestone beds of Slite. It belongs only to the higher 

 limestone beds of Gotland, possibly beginning at the top of b. In the Lower Silurian 

 of Esthonia at Borkholm, F. Schmidts stage 3, a variety of this species has been found, 

 only distinguished by its thinner shell and finer reticulation, the interspaces between 

 the callosities of the surface being nearly of the same, small size över the whole shell. 



2. Tryblidium unguis Lindst. 



Pl. I figs. 33— .37, pl. XIX fig. 2. 

 Tryblidium unguis 1880. Lindst. iii Angelin & Lm Fragmenta Silurica p. 16, pl. II, fig. 10 — 14, excl. fig. 15. 

 Shell of obovate outline, anteriorly acuminate, posteriorly expanded, with the 

 greatest breadth somewhat behind the median, transverse axis of the shell. It is re- 



') Oii sorae unicellular Algre parasitic witliiu Silurian and Tcrtiary Corals. Qu. Journ. Geol. Soc. 1876, p. 20.5. 

 •^) Ueber (las ausgebreitcte Vorkoramen von pflanzlicheii Parasiten iu den Hartgebilden niederer Tliiere. 

 Zeitschrift f. wissensch. Zoologie lOr Bd 1860, p. 215. 



