120 ANNALS NEW YORK ACADIJMY OF SCIENCES 



of the great wing of the s])hen()i(l and from the outer surface of the fex- 

 tcrnal i)tervooi(] plate. It is inserted in a depression in front of the neck 

 of the mandibular condyle and in the inter-articular fibro-cartilage and 

 capsule of the temporo-maxillary articulation. 



Pterygoideus in f emus. — Arises by two heads: one from the deep sur- 

 face of the external pterygoid plate and another by a stout tendon from 

 the tuberosity of the mandible. Both heads are inserted in a triangular 

 area on the inner, posterior surface of the mandible. 



MUSCLES OF THE DEPRESSOh' OR DIOASTRIC GROUP 



Digastricus (pars posterior). — The digastric as a whole arises in the 

 digastric groove of the mastoid process of the temporal and is inserted in 

 the lower, inner border of the anterior portion of the mandible. It is a 

 two-bellied muscle, the two bellies being separated by a stout tendon. A 

 slight tendon also attaches to the middle of the muscle to the hyoid bone. 



EECOXSTRUCTIONS OF THE JAW MUSCLES IN CEETAIN 

 EXTINCT VERTEBRATES 



Placodermi 



The great class of the Placodermi, including many S23ecialized forms, 

 apparently never attained true gnathostome jaws. Gregory says: 



All the known ostracoderms appear to be aVierrantly specialized in certain 

 directions, but long consideration of their many peculiar characters has cou- 

 riuced me that they stand far below the true fishes and that the group as a 

 whole may represent an important stage in the genealogy of the vertebrates. 



In tlie upper Silurian we have many forms, like Birkenia and Lasanius 

 of the order Anaspida, Lanarkia and TJielodus of the order Heterostraci, 

 without true gnathostome jaws, so far as has l)een discovered. The mouth 

 in these forms was probably a sucking type with a sphincter around it. 

 Dr. Gregory (1915) has the same idea: 



Of the many beautifully preserved specimens of Birkcnia, Laiiarkia, Drc- 

 panispis, PtcraHpiK, Trcmataspis, Ccphalispis and allied genera, none show the 

 least indications of an internal skeleton, nor is there ever any trace of the 

 brancliial arches and internal jaws. . . . The ostracoderms represent a 

 stage in chordate phylogeny immediately preceding the acquisition of a carti- 

 laginous skeleton impregnated with mineral salts : their first visceral arches, 

 if present, had not been transformed into primary or cartilaginous jaws : the 

 process of cephalogenesis was in a low stage and the elements of their shelly 

 exoskeleton were iiotentially homologous with cosmine, vasodentine and isopo- 

 dine of the primitive ganoids. To that extent they stand in a "pre-gnatho- 



