Vol. 54.] MR. h\ A. BATHER ON PETALOCRINdS, 417 



left in a plane inclined 45"^ to the verbical (P, P in text-fig. 5) ; 

 and it was perhaps in this way that the arms closed. 



In P. mirahilis the facet has a similar structure, but the petrifying 

 processes have not preserved it so perfectly. It is seen in specimens 

 e and b (PL XXYI, fig. 53), and its single axial canal is visible from 

 the interior of the arm-fan in that arm of specimen c which is 

 numbered iv. The better preserved crowns of this species confirm 

 the foregoing inferences from the structure of the facet. The 

 normal position of the armsis shown in d (PI. XXYI, fig. 38) ; they 

 spread out almost regularly from the cup, in such a way that no 

 part of the fan rises above the tegmen, but the convex curve brings 

 their distal ends fully 2 mm. below the base of the cup. In f 2 

 (PI. XXVI, fig. 37) the concavity of the crown in its dorsal aspect 

 is still greater: the width of the crown is 15 mm., and the arm- 

 fans bend to 2 mm. below the base ; the height of the cup being 

 2 mm., the total depth of the curve, from arc to chord, is 4 in 15. 

 In many specimens one arm-fan often overlaps another on one of its 

 sides : specimen g (text-fig. 1, p. 401) shows arm-fan i overlapping 

 arm-fan ii ; in specimen d (PL XXVI, fig. 38), arm -fan i overlaps 

 arm-fan v ; in specimen f 1 (PL XXVI, fig. 46), arm-fan iii over- 

 laps ii. The appearance suggests a screw-propeller, and in life this 

 resemblance may occasionally have been still greater. 



The opening of the axial canal in the facet is, as a rule, single 

 and circular, as in P. mirahilis, specimen e. But in P. mirahilis, 

 specimen b, the axial canal is rather wider than high, while in 

 the type of P. longus it appears as though split in two vertical 

 slits, which lie in a slight depression (PL XXVI, fig. 59). These 

 appearances are emphasized in two specimens of P. vishycensis 

 (i & m), the proximal ends of which have considerable interest. 

 Specimen i (PL XXV, fig. 21) may have been affected by weather- 

 ing, but of fracture there are no traces ; in it two of the ventral 

 grooves come right to the proximal end : in fact, one might even 

 say that four grooves come to the edge of the facet. Consequently 

 two axial canals are visible, underlying the two grooves of the 

 first dichotom. In the middle of this facet is a rather sharp 

 vertical depression, which corresponds to the middle of the median 

 ventral ridge, and is prolonged for 3 mm. on its upper surface 

 as a distinct, though slight and not quite continuous, groove. In 

 in (PL XXV, fig. 22) it is clear that the abnormality has not been 

 produced either by weathering or fracture, but that the proximal 

 end has a natural surface of union. This surface is shaped like 

 a facet, but broader (3 mm. wide, and 1*5 mm. from the bottom 

 of the grooves to the dorsal margin) ; and the angle which it makes 

 with the ventral surface of the fan is similar to that of an ordinary 

 facet. Four ventral grooves come right up to, and are cut short at, 

 its ventral margin, and below each of these opens an axial canal. 

 Dorsal to the line of the four axial canals is a very slight elevation, 

 hardly to be called a ridge, while between this and the outer rim is 

 a slight depression. It is probably a correct description of this 

 abnormality to say that the arm has broken off along the suture 

 between certain of the IIIBr ; and one infers from the regularity 



2g 2 



