RECORDS OF MEETINGS " 267 



other authors had mistaken the ventral for the dorsal surface of the skull 

 and the cotylus of the nuchal spine for the foramen magnum. Removal 

 of the hard matrix by etching has revealed the principal foramina and 

 other details of the numerous well-preserved skulls. The Pleuracanths 

 must be regarded as highly specialized rather than primitive Elasmo- 

 branchs. 



Mr. Nichols's paper was illustrated with lantern slides. 



Dr. Gregory reviewed some of the evidence which had led him to the 

 following conclusions regarding the evolution of the locomotive organs : 



(1) That myomeres, or contractile coelomic, mesodermal pouches are 

 the oldest and most essential part of the locomotive apparatus. 



(2) That the differentiation, concrescence and other modifications of 

 the myomeres have determined corresponding differentiations, concen- 

 trations, etc., in the nervous system, not vice versa. 



(3) That, with the possible exception of the notochord, the endo- 

 skeletal structures have all been determined as to their origin by the 

 arrangement and function of the myomeres and of the interjacent myo- 

 commas, not vice versa. 



(4) That the acquisition of a many-layered skin capable of secreting 

 hard deposits of calcified cartilage or of bone was a critical stage in the 

 evolution of the vertebrates, because it permitted the formation of exo- 

 skeletal structures (scales, surface bones, dermal rays), originally protec- 

 tive, which afterward became functionally connected with the locomotive 

 apparatus. The primitive scales themselves may represent highly modi- 

 fied sense organs. 



(5) That the earliest fins were mere folds of skin or ridges on the 

 body, serving as keels at nodal points, in connection with flexures of the 

 body. 



(6) That the myomeres were either originally or secondarily produced 

 into the fin-base and that rod-like cartilages were laid down in the con- 

 nective tissue areas between the myomeres. 



(7) That both the median and paired fins were originally broad -based, 

 the basal cartilages lying wholly within the body-line ; but as the fins ac- 

 quired independent motion, the basal cartilages became widely protruded, 

 changing the fins into the various paddles, either with a wide fin-web or a 

 reduced fin-web. 



(8) That uniserial or mesorhacic fins were independently evolved in 

 the Crossopterygii and Dipnoi and that the broad-based fins of other 

 fishes were in no sense derived from the mesorhachic type. 



(9) That the limbs of Tetrapods were evolved from paddles with 

 widely protruded basals which were of spreading or fan-shape, as in 

 Sauripteris. 



