Vol. 54.] ME. F. A. BATHER ON PETALOCEINTTS. 413 



The sides in P. vishycensis and P. angustus are not quite straight 

 or quite curved, but follow the contour of the branching arm 

 (PI. XX y, fig. 31). This is not obvious in the other species; but 

 in them the lateral edges are rarely well enough preserved to show 

 it, if it did occur. 



The ventral surface of the arm-fan, taken as a whole, is slightly 

 convex ; the amount of convexity varies in the species and, to some 

 extent, in individuals (PI. XXY, figs, 5, 12, 16, 30, 33, & Pi. XXYI, 

 figs. 51, 65). 



The dorsal surface has a slight concavity in the middle region, 

 corresponding to the ventral convexity ; but proximally and on all 

 the margins it is convexly rounded towards the edges, except where 

 the facet comes. In P. vishycensis portions of the dorsal surface 

 are often raised in irregular lumps or swellings, apparently due to 

 a deposit of stereom secondary to that of the original brachials (see 

 text-fig. 6, p. 418). This secondary stereom is often arranged in lines 

 of growth, vaguely concentric with the distal margin (as seen in 

 specimens c, d, e, PI. XXY, figs. 7 & 29), and it is covered with a fine 

 shagreen ornament (well seen in c, e, g, and j, PI. XXY, fig. 23). 

 Towards the distal margin the secondary stereom is less thick or 

 entirely absent, and the original branching of the arms is often 

 shadowed forth on the back by slight depressions ; this is well 

 shown in the young specimen g (PI. XXY, fig. 2), and it is here 

 seen that the depressions correspond with the ridges on the ventral 

 surface. The secondary stereom appears first on these depressions ; 

 its successive layers did not cover the whole of the preceding ones, 

 but each layer was more distal, so that an imbrication is visible in 

 the lines of growth, the later and more distal layers overlapping 

 the earlier proximal ones. On the dorsal surface of specimen e 

 (PL XXY, fig. 7), near the distal margin, is the valve of a small 

 Beyrichia ; this appears to have become partially overgrown around 

 the edges by the secondary deposit. The dorsal surface is not 

 visible in any other species, except P. mirabilis and P. lonyus, 

 and in them there is no evidence of growth-lines or shagre^i 

 ornament; such irregularity as occasionally occurs seems due to 

 mineralization and weathering. 



Further evidence of original free branching is afforded by the 

 distal margin. This, in many specimens, instead of forming a 

 regular curve, is slightly excavate or depressed, either in the medial 

 region between the two halves of the arms (as, for example, 

 P. vishycensis, specimen f, PI. XXY, fig. 4 ; P. mirabilis, specimen 1, 

 PI. XXYI, fig. 52; P. e.vj)ansus, Pi. XXY, fig. 34; P. angustus, 

 PI. XXY, fig. 31), or in each half, in the axis of the second dichotom 

 (as, for example, P. vishycensis, specimen w), or more irregularly 

 (as P. vishycensis, specimen b). Thus a slightly-lobed outline 

 is often seen in P. vishycensis, but hardly at all, and never to so 

 great an extent, in P. mirahilis. This lobation is merely an exao-- 

 geration of what is seen in many specimens of P. vishycensis and 

 some of P. mirahilis, namely, fine indentations at the distal ends of 

 the ridges. This is not to be confused with the slight crenulation 

 or scallopping of the distal edge due to the accentuation of the 



