104 WHITEAVES ON FOSSIL FISHES FROM 
convexly outward and backward in such a way as to leave a broadly triangular opening 
with concave sides, though the posterior and larger part of the opening and a very small 
portions of the plates themselves are apparently overlapped by the supposed hyoid plate 
(No. 18), as represented in the diagram on Plate VII. The posterior edge of each of 
these plates (No. 15) is smooth and rather broad, but the larger and anterior portion is 
sculptured like the rest of the dermal plates. 
The pectoral spines extend a little beyond the centre of the posterior margin of the 
dorsal shield, but not beyond that of the ventral. They are thin and flattened vertically, 
so that the outline of a transverse section of one of them above the middle would be very 
narrowly wedge-shaped, with the thickest end of the wedge inward. They are divided 
nearly transversely, below the middle, into two segments of very unequal size, by a ball 
and socket joint, the ball being in the anterior, and the socket in the posterior or terminal 
segment. "he anterior end of each pectoral is also furnished with a ball and socket joint, 
there being a strongly inflected or excavated cavity in the ventrolateral plate (No. 19) of 
the ventral shield, to which the anterior end of the spine, which terminates in a rounded 
protuberance, is articulated The anterior segment, which is much broader and longer 
than the posterior, is of nearly equal breadth throughout, but the posterior segment is much 
more slender and narrows somewhat rapidly to an acute point. The outer edge of the 
whole of the pectorals bears two or more rows of close-set, short and conical, hollow spines, 
and there is a similar but apparently single row of spines, on their inner edges also. Both 
segments of the pectorals are divided into numerous plates, of the shapes and numbers 
indicated in the diagrams on Plates VI and VII, the plates of the anterior segment being 
large and comparatively few in number, while those of the posterior are smaller and 
much more numerous. 
The whole of the outer surface of the cranial, dorsals and ventral shields, and that of 
the pectorals, is sculptured in a very complicated way that is difficult to describe, but of 
which Plates VIII and IX, which are taken from photographs, give a very good idea. 
This ornamentation consists essentially of closely aggregated and minute shallow pits, 
surrounded, or partially surrounded, by raised ridges, composed of confluent tubereles, or 
by rows of tubercles which bend, curve, or divide in almost every direction, and anastomose 
with or cross each other in such a way as to form a very irregular and minute areolation, 
or more or less incomplete network. Each of these little pits (which Agassiz has compared 
to the marks that might be made by pressing the head of a round-headed pin lightly into 
fresh plaster) is perforated by from one to three minute, vertical and circular canals, which 
pass through the entire thickness of the test, and which are easiby seen by the aid of an 
ordinary simple lens. 
In some large and presumably adult if not aged individuals, in addition to the sutural 
lines and the ordinary areolation, a widely divaricating impressed line runs forward and 
outward from the centre of the posterior margin of the cranial shield to a point on each 
side of the head, a little in advance of the orbital opening. A similar, but rather longer, 
divaricating impressed line, also runs outward and backward from the centre of the 
dorsomedian plate to the upper and outer limits of the tail opening. 
In a cast of the interior of the head and body shields of a single specimen, there are 
obscure indications of a pair of flattened, sigmoid, and outwardly directed processes on the 
anterior margin of the cranial shield, which the writer once thought might be analagous to 

