164 



SILURIAN RADIATA. 



boundary, over which they frequently pass into the adjoining 

 stars : vertical section showing a few deUcate vertical striae 

 under the centres of the stars (edges of the twisted vertical 

 lamellse), and a nearly uniform small vesicular tissue, the cells 

 of irregularly unequal size, passing from star to star, with a 

 few irregular dense bands nearly coinciding in curvature with 

 the form of the cells ; average of larger cells two to three in 

 1 line; no divisional walls between the stars. 



That this is one of the fossils figured by Mr. Lonsdale in the 

 ' Silurian System ^ as the Acervularia Baltica of Schweigger I 

 have little doubt, but neither the particular figure to which I re- 

 fer, nor the others which that writer has given under the same 

 name in that work, have any specific relation to the Acervularia 

 Baltica of Schweigger, who avows that name to be only a syno- 

 nym of the A. ananas (Linn, sp.), referring to the original figure, 

 &c. in the ' Amoenitates Academicse ' of Linnaeus, representing a 

 coral which has a distinct-walled, tubular, centre, from which the 

 groups of young are developed, solid walls between the cells, &c. 

 The lateral disk buds are often seen in this curious coral. 



Wenlock limestone near Aymestry. 



{Col. University of Cambridge.) 



Petraia cequisulcata (M'Coy). 



Sp. Char. Conical, slightly curved, oblique, with a few broad, ob- 

 tuse, undefined concentric swellings of growth; regularly in- 

 creasing from the apex to a diameter of 1|^ inch at 2| inches 

 from the base ; external wall very thin, it and the cast regularly 

 marked with equal obtuse ribs, about eighty-five in the adult 

 diameter of 1^ inch (six in 3 lines), eighty at 1 inch (eight in 

 3 lines), seventy-five at 9 lines and forty- six at 4 or 5 lines 

 (ten in 3 lines), separated by thin, equal, equidistant slits 

 representing lamellae, each alternate one of which is merely 

 marginal, the others occasionally and regularly uniting before 

 reaching the centre, round which they are twisted to form a 

 spirally conical central area ; no connecting vesicular plates. 



The regularity and equality of size of the lamellar ribs both 

 on the exterior and on the cast, as well as their large number, 

 give a peculiar aspect to this species. In parts of some speci- 

 mens the lamellar sulci are bent in a zigzag manner, but it is ah 

 unusual appearance, for which I cannot account. By carefully 

 removing part of the outer wall, each of the lamellae is seen to be 

 split near the exterior (not visible on casts), and each to be per- 

 forated by tubuli, as in Mr. Lonsdale's supposed genus Try- 

 plasma; these punctures leave no trace on the equal, obtuse, 

 smooth ridges of the cast. I have usually seen this coral ticketed 



