52 G. LINDSTRÖM, ON THE SILUUIAN GASTROPODA AND VTEKOPODA OF GOTLAND. 



tion (lig. 23) they form u nearly right ungle. Tlic surface is deeply grooved by 

 transverse ridges, coincidiiig with the liiies of growtli and separated from each 

 other tlirougli narrovv interstices. They exactly follow the same directiou as tlie out- 

 lines of the aiiterior 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 mai'gins. There is a central area formed by two 

 hiteral, lougitiidinal grooves, which run nearer to the kxteral margin than to the cen- 

 tral ridge and converge towards the posterior apex. These grooves are conspicuous 

 in a few specimens, being effaced by corrosion in the othei-s. The exteriör 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, first 

 the posterior apical area, which is of highly vai-ying 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, fig. 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 tnmid. No traces of muscular impressions are visible- 



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

 C h. 18 raillim., 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 Grötlingbo 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 liaiestone above 

 »Kålens qvarn» near Wisby. 



Fara. II. PATELLID^ d'Orbigny. 

 Gen. TRYBLIDIUM Lindström. 



Syn. 1836. Metopluma Pu]Li,n's p. p. Geol. of Ynrkshire pt. 2 p. 223. 



1880. Tryblidhim Ldm apud Angelin & Lindström Fragmeuta Silurica p. 15. 



Sliell ■patelliform, obovate, anteriorly acuminate, posteriorly eniarged, forming a very 

 low cone. Apex anterior, nearly marginal, ivith only very little area heneath. Margin of 

 the ovate aperture arched, so that the animal, ivhen fixed, was not entirely hidden heneath 

 its shell. Muscalar scars in six, disconneded pairs, arranged in an oblong circle, opien or 

 nearly so towards the front part of the shell. Intiiiiate striicture of the shell souiewhat 

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



This new genus, which I at first described in Angelins and my work »Fragmenta 

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



