Yol. 54.] MR. F. A. BATHER OT>f PBTALOCRINUS. 433 



be harmonized with the law for the genus, and especially in the 

 fact that the right-hand branch arising from the left IIlBr, forks 

 again so soon as almost to produce the appearance of a trichotomy. 

 The interpretation consequent on thrice-repeated examination of 

 this particular area results in the numbering of the finials from 

 left to right as Y, Y, Y, YI, YI, lY, lY ; this does not agree 

 with either of the other portions in any respect, nor is such an 

 arrangement found in any normal half of an arm-fan. Matters are 

 not made clearer by any other interpretation that might possibly 

 be put on portion M. 



One may, therefore, describe the peculiar structure of this arm- 

 fan as due to the intercalation of an abnormal portion (M) between 

 two normal halves (L and E,), M being derived by the abnormal 

 branching of IIBr in L. This of course is no explanation, and it is 

 indeed difficult to conceive a cause that could produce such variation 

 in a structure so compact and usually so symmetrical, especially as 

 the superficial symmetry of outline has not been disturbed. 



Although the branching of the right-hand half is theoretically 

 almost identical with that of the left-hand half, yet a practical 

 difference is obvious in that so many of the dichotoms begin at the 

 same level. There are in fact four adjacent finial dichotoms, three 

 in this half and one in the intercalated portion, all of which start 

 almost on the same line; one result of this is a sudden and marked 

 thinning of the ridges in this region. The ridges are finely 

 crenulate, and owing to the narrowness of some of the ridges, the 

 crenulations meet right across, so that the edge is almost saw-like. 

 In portion M, near the proximal end of the long finial, third from 

 the left, there appear to be a few covering-plates preserved. They 

 are inserted in the crenulations, and are much depressed. In some 

 ridges there is a slight projection or rabbet below the crenulations, 

 and this probably served to support the covering-plates. The axial 

 canals are exposed at the distal ends of the grooves, as shown in 

 PI. XXVI, figs. 63 & 64. 



The ventral surface of the arm is almost flat, but bends down- 

 ward at the proximal end (PL XXYI, fig. 6d). 



The distal edge is verv slightly scalloped at the ends of the 

 grooves, while the ends of the ridges are almost flat, and do not 

 show the slight indentation observed in P. mirabilis. 



The dorsal surface (PL XXYI, fig. 58) is almost flat, rounded off 

 along the two sides, especially at their proximal ends. It is covered 

 with curious raised markings, like flat, irregularly anastomosing 

 bands. These are probably due to mineralization and weathering. 

 There are no traces of lines of growth or of shagreen ornament. 



The facet (PL XXYI, fig. 59) is semicircular, widening outward 

 at the upper angles. The axial canal, which lies in the middle, is 

 split into two vertical slits, and lies in a slight depression, on either 

 side of which, dorsad of the canal, is a scarcely perceptible swelling, 

 corresponding to the two ridges in P. vishycensis. 



Relations to other Species. — In the angle of the arm-fan 

 P. longus resembles F. anqustus ; but, even apart from its greater 



2h2 



