52 G. LINDSTKOM, ON THE 8ILUKIAN GASTKOPOIJA AND PTEKOPOUA OF GOTLAND. 



tion (lii;-. 23) they t'orin a nearly right angle. The surface is deeply grooved by 

 transverse ridges, coinciding with the lines of growth and separated from each 

 other through nari'ow interstices. They exactly follow the same direction as the out- 

 lines of the anterior margin, being bent backwards at the lateral margins, then for- 

 wards and on the middle of the back again backwards, thus forming a sigmoid line. 

 They are most conspicuous near the margins. There is a central area formed by two 

 lateral, longitudinal grooves, which run nearer to the lateral margin than to the cen- 

 ti'al ridge and convei'ge towards the posterior apex. These grooves are conspicuous 

 in a few specimens, being eftaced by corrosion in the others. The exterior sur- 

 face is moreover ornamented by numerous, minute wartlets, which are closely set 

 in regular, transverse rows (tig. 15). On the inside of the valves there is seen, lirst 

 the posterior apical area, which is of highly varying width, in some occupying nearly 

 half the length of the surface, in others only a fourth (fig. 25) just as in a recent 

 Chiton, of which a figure is given on plate II, iig. 28. Owing to the state of con- 

 servation of the specimens, the lines of growth are more obscure than in the former 

 species. The whole inside is longitudinally divided into two halves by a deep sinus; 

 the sides being more scooped out at the apical area, the side parts of the anterior 

 surface being more tumid. No traces of muscular impressions arc visible- 

 Specimen A height 18 millim., breadth 14 millim., B h. 11 millim., br. 12 millira., 

 C h. 18 millim., br. 11 millim. In a specimen of 14 millim. in length, the greatest 

 thickness of the plate is 3 millim. 



This species is more common than the preceding and has been found in several 

 specimens at Grotlingbo in the oolite quarry near Gannviken, in the sandstone of Burs- 

 vik, in the oolite near Rone and also in several detached plates in the limestone above 

 "Kalens qvarn» near Wisby. 



Fam. II. PATELLIDiE d'Okbignv. 

 Gen. TRYBLIDIUNI Lindstuom. 



Syii. 1836. Mcloptoiiia riiiLi.u's p. p. Gcol. of Yi rkshire pt. 2 p. 223. 



1880. Trijblidium Ldm apud Axuelin & Lindstrom Fragmeuta Sikuica p. 15. 



Shell paUdliforin, obovate, anteriorly acuminate, posteriorly enlaryed, forviint/ a very 

 low cone. Apex anterior, nearly marginal, with only very little area beneath. Margin of 

 the ovate aperture arched, so that the animal, when fixed, was not entirely hidden beneath 

 its shell. Muscular ncars in six, disconnected pairs, arranged in an oblong circle, open or 

 nearly so towards the front part of the shell. Intimate structure of the shell somewhat 

 rescndjliiig that of the recent Patella, being composed of thin strata of polygonous cells. 



This new genus, which I at iirst described in Angelins and my work "Fragmenta 

 Silurica» p. 15, had by some palaeontologists long been considered as belonging to the 



