112 G. LINDSTRÖM, HELIOLITID^. 



It coijsists oiily of a thiu iilm, covered by a great nurnber of rouiided, glossy tubercles, 

 confusedly arranged in squares as if to foreshadow the outlines of calicles and within 

 them heaps of smaller tubercles in lines following the costa; and striae on the surface of 

 the subjacent shell. 



The mature coral has eircular calicles of 2 millims. in diameter constantly with 

 twelve, comparatively thick septa, the crest of which is indented with a single row of 

 tubercles. They leave a narrow area free in the centre, covered with a few blunt tubercles. 

 Between the closely packed calicles there is very little space left for a coenenchyma, but 

 a narrow surface covered with tubercles without any pores or tubuli may be indicated as 

 a sort of coenenchyma. 



The sclerenchyma is made up of the distinguishing large baculi of pinnate structure 

 and there is not the least vestiges of tabulfe or dissepiment. 



There is a sort of coenenchymal gemmation, as new calicles arise ont of the 

 surface between the calicles. 



This is an exclusively Lower Silurian coral and the oldest known of this family as 

 it is found in the strata of Wesenberg in Estland. But it also ranges higher to the 

 Lyckholm beds of Estland, at Piersal and to the Leptsena Limestone of Dalecarlia. In 

 North America it is very common in the Trenton limestone, in the Cincinnati group, in 

 Ohio and at Richmond, Indiana. 



Gen. Acantholithus n. 

 ('Axäv&a, spino.) 



Corallum grown in tliin lamellie, several close above each other linked together on 

 one side, each of the superior ones being in organic connection with the next inferior 

 or growing from a corner of them. The inferior surface formed by a uncommonly thick 

 epitheca of tubular structure. The superior surface with the calicles, which as a rule 

 have short septa surrounding a central area, covered with numerous short papilla?, which 

 are the heads of the columnar baculi. There are no tabulse in the calicles, nor are these 

 circumscribed by a well defined theca. The coenenchyma consists of very narrow tubuli, 

 with thick walls of irregularly polygonal or eircular shape. These walls show the same 

 pinnate structure as the baculi. In some specimens there are tabulaä, concave, distantiated, 

 but as a rule the walls have increased in thickness as to leave no place for the develop- 

 ment of such. This genus which is chiefly Lower Silurian has affinities on the one side 

 with Cosmiolithus through the tinely reticulated coenenchyma and on the other hand, with 

 the Coccoseridaj through the well developed baculi and the whole conformation of the calicles. 

 There is a large material still to be found in the Lower Silurian strata of Estland, which, 

 when collected, may clear up some obscure points in the structure of these beautiful corals, 

 as yet difficult to explain. 



