56 G. LINDSTUOM, ON THE SILQIUAN GASTUOroDA AND I'TEUUl'ODA 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 interior one, which is 

 yellow. The later is very thin, consisting of minute lamellae of an ap])arently prismatic 

 structure. A singularity in structure is that figured on pi. Ill f. 4, a longitudinal 

 section, showing the extremely thin lamella) perforated, as it were, by narrow tubes. 

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

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

 laminae are bent exactly where the tubes are situated. 



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

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

 the surface is pitted by a number of irregularly placed, cream-coloured points (pi. Ill 

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

 white dots are seen to prolong downwards through the lamellae of the shell as tubes 

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

 lets (pi. Ill 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 (pi. Ill 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. Kollikeh ') has also more in full 

 described a great number of such minute, i)arasitic 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 fungi. The white radiating tubuli in the shell- 

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

 then to have been filled with limestone 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 Fiir6 at Lansa, Lutterhorn, in the 

 limestone of Wialmsudd near Farosund, Svarfvare huk, in the canal at Westoos 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 Borkholra, F. Scumidts stage 3, a variet}' of this species has been found, 

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

 the callosities of the surface being nearlv of the same, small size over the whole shell. 



2. TrylDlidium unguis Lindst. 



PI. I liKs. 33—37, pi. XIX fig. 2. 

 Trijblidium tinguis 1880. Lindst. in Anoei.in & Lm Fragmenta Siliirica p. 16, pi. II, fig. 10 — 14, exci. fig. 15. 

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

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



') On some unicellular Algte parasitic within Silurian and Tertiary Corals. Qu. .Fourn. Geol. Soc. 1870, p. 205. 

 -) Ueber (las ausgebreiteto Vorkoramen von pflanzlicheii Parasiton in ilrn llartgebilden niederer Tliiere. 

 Zcitschrirt f. wissensch. Zoologie lOr Bd ISGO, p. 215. 



