KONGL. SV. VET. AKADEMIENS IIANDL. BAND. 19. N:0 6. 55 



1. TryTblidium reticulatum Lindstr. 



n. I figs. 25—31, pl. III figs. 1—5. 



Tr. reticulatuiii. 1880. LiNDST. in Anoeltn & Lindström Fragraenta Silurica p. 15, t,ab. X, lig. 7 — IC.tab. XIX, 

 (ig. 9—12. 



Shell miich depressed and gencrally more flat than the following. Apex obt.usc, 

 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 acurninate, enlarged towards 

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

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

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

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

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

 spouding 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 exteriör stratum of the shell is there worn away, making the interiör yellow 

 stratum alone visible. Sometimes there is the false appearance as of a forainen, 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 börder. When 

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

 hio^hest along the raiddle of the shell and sinkino; towards both ends. The foot of the 

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

 On the interiör surface, which is very smooth and almost glossy, there 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 interiör margin and at its exteriör margin they are lobate or lacerated. The 

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

 most is elongated, elliptical, the exteriör one irregulai"Iy 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 interiör börder 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 consists of two distinct strata, of which the 

 exteriör one easily peels off and the interiör, in specimens from certain localities, is 

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

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



') Fragm. Silur. pl. XIX fig. 9. 



