230 DESCRIPTION OF THE SPECIES OF PTERYGOTUS 
the animal. The figured specimen, 2, is the same as that figured by 
Agassiz, and is nearly three inches from back to front, but we have 
other specimens which measure four inches and a half long, and were 
probably sixteen or eighteen inches wide. 
The “ seraphim,” as the piece is called, is straight or only sinuous 
in front, but broadly wedge-shaped behind ; its extremities are un- 
known, unless fig. 6 be the lateral termination on one side. It 
consists of a central arrow-shaped piece and two widely-expanded 
wings or lateral lobes which project backwards nearly as far as the 
shaft or central lobe, forming with it the broad salient angle of the 
posterior margin. These lobes are not, however, united with the 
shaft but overlap it with thickened edges on each side, giving it an 
appearance of contraction in the middle; it is, when detached, a 
linear piece, somewhat dilated at the base, convex along the median 
line, deflexed at the sides, and expanding at the base into a broad 
arrow-headed plate, the point acute, subtending a variable angle 
(generally about sixty degrees), and reaching nearly to the front. 
This portion is divided from the general surface by sutures, but not 
quite separable. The apex of the central lobe is expanded, rounded, 
and marked with radiating plaits. As in all other parts of the 
crust the plicaze which closely cover the surface are transverse and 
but little curved in front, but become semicircular further back, 
and then semi-ellipses, and even cones, closely tiling over one an- 
other. It is this feather-like arrangement, combined with the shape, 
which must have suggested the term “seraphim.” The extreme 
margin is smooth. The plice on the arrow-headed central piece of 
the labrum follow the same order from before backwards, and become 
elongate and prominent along the central ridge, forming tesselle 
rather than scales. 
Body Joints, Plates IV. and V. [Plates 15, 16]—The segments 
of the thorax and abdomen may easily be arranged in their right 
order, from comparison with those of the preceding more perfect 
species. Plate IV. [Plate 15] represents all the anterior segments 
we had observed when the plates were engraved. But since their 
completion, a cast (agreeing very nearly with the figures in Plate ITI. 
[Plate 14]) of a fine specimen of the anterior body rings, and nearly 
a foot wide, has been sent up by our correspondent, Mr. W. Miller,. 
of Dundee. It is from a new quarry at Tealing, Forfarshire. The 
specimen is in the Museum of the Watt Institution, Dundee, and a 
woodcut, one-fifth the natural size, is given of it in p. 231. From the 
second to the ninth, the segments are all united, and the anterior ones 
have slipped a good deal over one another so as not to present quite 
their full dimensions. 
