STRUCTUKB OF CEBTAIN PALEOZOIC DIPNOI. 209 
and passing very obliquely backward. The bone is continued forward for a 
considerable but uncertain distance in contact with the inner surface of the 
pre-articular. ' 
The pre-articular is the bone usually called splenial in Ceratodus. In 
Dipterus platyceplialus it is of "great length, extending from the articular 
facet to a symphysis with its fellow, which extends forwai'd nearly to the 
anterior end of the jaw. 
Its hinder part is a vertically placed thin sheet of bone, tightly applied to 
the inner surface of the articular, with a free upper margin which forms the 
border of the supra-Meckelian vacuitj% and with its lower border in contact 
with a depressed strip of bone which appears to be a part of the angular, but 
is perhaps, as Dr. Stensio has suggested to us, really the ossified Meckel's 
cartilage. It is possible that in the naturally articulated jaw the strip was 
completely •covered by the pre-articular, and that in consequence the ramus is 
represented as of too great a depth in fig. 34. 
From a point not far in front of the articular to the hinder end of the 
dentary the upper edge of the pre-articular is turned outward and the bone 
thickened ; anteriorly this edge meets the angular, forming the border of the 
very large supra-Meckelian vacuity. 
This out-turned edge of the pre-articular bears the tooth-plate, which is a 
thick pad with denticulated ridges radiating from a point on the inner border 
rather behind the middle. The pattern of these teeth-plates diffeis consider- 
ably in individuals ; they commonly extend much further forward than in 
fig. 34. 
The pre-articular is completed by turning inward as a flat, nearly horizontal 
plate to the symphysis, which lies on the dorsal surface of the spleuials. 
In the reconstructions represented in fig. 34 the ramus mwj be made too 
deep and wide. The depth illustrated depends on the actual depth in the 
specimen ; the width is fixed by that, of the hinder end of the dentary and of 
the tooth-bearing part of the pre-articular. At the most, the inaccuracy can 
only be of the order of one or two millimetres. 
The course of the lateral-line canal on the lower jaw is well shown in the 
specimen. Along the lower border of the angular it is represented by an 
irregular and oiten double series of very small pits. 
On the post-splenial are two definite lines of large pits, one passing 
forward parallel to and just within the lower margin of the lateral depression, 
the other along the hinder border of the bone ; in addition, there are other 
scattered pits. The splenial has two pits near and parallel to its hinder 
marginr 
The new information about Dipterus recorded above adds very con- 
siderably to the evidence in support of the view first definitely stated by 
L. Dollo that Dipterus is by far the most primitive, as it is the oldest 
known Dipnoan. 
