KONGL. SV. VEl'. AliADEMIENS HANRL. BAND. 19. N':0 6. 55 



1. Tryblidium reticulatum Lindstr. 



PI. I figs. 25—31, pi. Ill tigs. 1—5. 



Tr. reticnlatuiii 1880. Lindst. in Angei.in & IjINDstrom Fragment:! Silurii-a p. 15, tab. X, lig. 7 — 16, lab. XIX, 

 lig. 9—12. 



Slu-U much depressed and generally more Hat than the following. Apex obtuse, 

 erect, with a narrow, area-like zone between itself and the margin of the shell. Out- 

 line of the shell, when seen from above obovate, anteriorly acuminate, enlarged towards 

 the posterior margin. The surface is ornamented by a series of thick, transverse and 

 concentric, elevated lamina', which in the anterior, third part of the shell intercross 

 with each other and thus form an elegant network, but in the other two thirds of the 

 surface are parallel, wavy, thin at the edges, thickejiod towards their base, and evi- 

 dently formed by the margins of the shell bending upwards. In the oldest part, corre- 

 sponding to the shell of the young, the callous, transverse lines are very fine and 

 crowded. Around the apex there is almost always an oblong space more or less ex- 

 foliated. The exterior stratum of the shell is there worn away, making the interior yellow 

 stratum alone visible. Sometimes there is the false appearance as of a foramen, which 

 has been filled up again, but this is evidently owing to imperfect corrosion. 



The aperture is obovate, the margins very thick, forming a callous border. When 

 seen from the side the line formed by the margin is an elongated curve or an arch, 

 hiofhest alonji the middle of the shell and sinking towards both ends. The foot of the 

 animal, when resting on its basis, must then have been left uncovered along the sides. 

 On the interior surface, which is very smooth and almost glossy, thei'e is a dark co- 

 loured circle, formed by six pairs of muscular scars, of which the foremost are the 

 largest and nearly connected by narrow appendages, like stripes, almost as in the recent 

 Olana cochlear. See plate I figs. 30, 32. Each separate scar is more elevated towards 

 its curved interior margin and at its exterior margin they are lobate or la(;erated. The 

 foremost pair is the largest, composed as it were, of several parts, of which the inner- 

 most is elongated, elliptical, the exterior one irregularly square; from its anterior 

 margin there projects, from each pair, a narrow stripe; these stripes do not unite, 

 but leave between them a short space. The central surface, enclosed by the muscular 

 ring, is excavated in shallow, arched depressions all along the interior border of the 

 scars, being situated in the interspaces between these. The scars themselves are 

 separated through narrow projections from the enclosed surface. Sometimes there are 

 large concentric callosities') due to the irregular growth of the margin, which cross 

 the central field. 



There are specimens still retaining traces of the original colour of the shell in 

 dark longitudinal stripes, distinctly conspicuous on the other dull surface (f. 29). 



As to its intimate structure the shell consi.sts of two distinct strata, of which the 

 exterior one easily petds off and the interior, in specimens from certain localities, is 

 the only one left on the nucleus of the shell. The exterior stratum is thick, attaining 

 as much as 5 millimeters in some parts. It consists of very thin lamina^ which seem 



') Fragm. Siliir. pi. XIX (ig. 9. 



