210 PKOF. D. M. S. WATSON AND MK. E. L. GILL ON THE 
Dipterus is now shown to resemble the Osteolepids in the following- 
ways : — 
1. In the typical cosmoid structure of its scales. 
2. In the general body form, possession of two separate dorsal and an 
anal fin, and a heterocercal tail with an epichordal lobe. 
3. In the number and relations of the dermal bones of the top of the 
head. 
4. In the presence of splenial and post-splenial bones in the lower jaw. 
5. In the possession of a very elaborate opercular apparatus, including 
opercular and sub-opercular, a series of lateral giilars, a pair of 
principal guliirs, and a median gular. 
fi. In the structure of its paired tins. 
These resemblances are of a fundamental character, and imply a community 
of origin of the two groups. 
The principal features in which Dipterus differs from an Osteolepid are: — 
1. The loss of the hyomandibular as a bone pihiying a part in the support 
of the quadrate. 
2. The suppression of the suprapterygoid ossicles and the possession of 
a symphysis between the pterygoids anteriorly, and of n sutural 
attachment of the pterygoids to the edges of the parasphenoid. 
3. The loss of the palatine and ectopterygoids, and the development of a 
" tooth-plate " on the pterygoid. 
4. The loss of the coronoids and the development of the tooth-plate on 
the pre-articular. 
5. The great reduction of the marginal tooth-bearing bones, the pre- 
maxillfe, maxillse, and dentaries. 
6. Tlie forward inclination of the quadrate, with a correlated shortening 
of the lower jaw and a great reduction of the cheek-plates. 
7. The removal ot the external nares from a point on the upper surface 
to a position below the lip. 
8. The absence of a movable joint between the parietals and frontals, 
and of an unossified region of the basis cranii between the basi- 
occipital and basisphenoid. 
M umbers 1-6 of these differences are obviously dependent on different 
food habits, the Dijynoi having adopted a diet which required trituration, 
whilst the Osteolepids are predaceous, swallowing their prey entire. 
Difference number 7 may perhaps have arisen not- as an adaptation, l)ut as 
a mere result of the mode of development of the olfactory organ in a fish 
which has a much reduced maxilla. 
The differences recorded under No. 8 can only be explained by supposing 
these peculiar characters to have been acquired by the Osteolepids after their 
separation from the Dipnoi. 
