I2O WILLIAM PATTEN. 



axis, thus swinging their ventral edges forwards and backwards 



o o o 



in a nearly horizontal plane. At the same time their median 

 ends could be thrown forwards and medianly, thus bringing their 

 diverging median margins more nearly parallel, and nearer 

 together in the median line. The difference in curvature, den- 

 sity and rigidity between the mandibles and maxillae, and the 

 direction of the marginal spines on the mandibles and their ab- 

 sence on the maxillae makes it very improbable that one pair 

 acted directly against the other. It is more likely that the 

 maxillae merely pushed the food forward and inward, where it 

 could be crushed, or cut, between the stout margins of the man- 

 dibles, in a manner similar to that which obtains in the mouth 

 parts of arthropods. 



I regard the paired upper and lower jaws of Bothriolepis as 

 the homologues of separate right and left halves of the embry- 

 onic mandibular and maxillary arches of the higher vertebrates. 

 As is well known these ridge-like lobes appear in all vertebrate 

 embryos on the sides of the head, close to the nerve cord and 

 then migrate toward the median ventral side. These embryonic 

 mandibular and maxillary lobes I have for many years regarded 

 as the vestiges of arthropod appendages which, owing to the 

 special conditions under which the vertebrate head is developing, 

 are carried toward the haemal side of the head, instead of the 

 neural side. 



Bothriolepis is the only fish-like animal in which the halves of 

 the jaws are separate, and functionally independent in the adult, 

 and it thus supplies precisely the condition which the arthropod 

 theory of the origin of vertebrates demands. 



Back of the maxillae, the oral membrane is strengthened by 

 two bands of dermal armor. They are very thin and delicately 

 ornamented, and when the head is in a normal position the pos- 

 terior band is entirely, and the anterior one partly, overlapped by 

 the anterior ventral plates. The narrow anterior band consists 

 of five or six segments. The posterior one is unsegmented. 

 Both bands are attached to a large lateral plate, the lateral end 

 of which is bent at right angles, and attached to the lateral walls 

 of the head, Fig. 3. 



A small quadrilateral plate, that I shall call the prc-Iatcra!, lies 



