58 (i. LINDSTRÖM, ON THE SILURIAN GASTROPODA AND I"l'ER()P()DA OF GOTLAND. 



3. Tryblidium? radiatum n. 



Pl. XVIII fig. 1, 2. 



Shell oval, inuch elevatedlj^ conical, the greatest height lying near the middle of 

 the longitudiiial axis. The apex anterior, truncated, curved as to lean forwards över 

 the anterior inai^gin of the aperture. The shell between the apex and the margin is 

 consequently much concave. The apertural börder is straight and horizoutal. The 

 surface is covered by fine, closely packed, radiating stritu and a few concentric lines 

 of growth. The shell has been thick, as may be seen by a few fragments which are 

 left along the borders. The general shape of this shell has led me to place it in this 

 genus, with the other species of which it corresponds as to its outline. This is, however, 

 more elevated like a Helcion. 



Diameter from the apex to the hind edge 24 mill. The transverse diameter 20 

 raillim. Height from the rim of the aperture 15 millim. Length of the aperture 

 23 mill. 



Only a single specimen has been found in the crystalline limestone of Wialrasudd 

 near Fårösund. 



Fam. III. TECTURIDiE Adams. 

 Gen. PAL/EACWI/EA Hall. 



1873. 23d Rep. on the State Cab. of N. York p. 242. 



Shell patelliform, aperture ohlongate, exteriör surface concentrically wrinkled^ cm the 

 interiör surface near the top a wreath of muscular impressions, neaHy coherent. 



It is only provisionally that the Gotland species, described below, can be placed 

 within this genus. It is chiefly in consequence of its outward shape, which most re- 

 sembles that of Palgeacmtea. In Hall's specimens the muscular markings are unknown. 

 The Patelloid shells which are now and then found in the older palteozoic strå ta belong 

 probably to several different genera. The »Patella» antiquissima from the Lower Silu- 

 rian of Sweden is nearly related to Tryblidium through its series of six pairs of de- 

 tached muscular scars. The typical species of Metoptoma, which as De Koninck has 

 shown have a coherent muscular band and consequently cannot, as Hall 1. c. has hin- 

 ted, be plates of a Chiton, agree with Lepetopsis in the conformation of this band. In 

 the Red Orthoceratite limestone of Öland at Wickleby Hr von Schmalensee has found 

 a specimen resembling a Metoptoma and another form probably belonging to the same 

 generic group of which Wiiitfield has given figures in Geology of Wisconsin, vol. IV 

 pl' 3 f. 15, 17, 18, but which scarcely can belong to Metoptoma as he assumes. The 



