296 Dr. H. Gaclow and Miss Abbott. On the [June 21, 



bone on its outer side, which corresponds to the distal end of the 

 ulna, but there is no trace of a third bone preserved, and there is only 

 one central bone preserved. There are three phalanges in a digit. 

 The femur is l^ inches long ; its articular head appears to be small 

 and hemispherical. There is a large internal trochanter extending 

 down the shaft, which corresponds with the similarly placed ridge 

 in the femur of Megalosaurs and other Saurischia. 



The slender character of the ribs, which are different from those in 

 known Theriodonts, suggests the possibility that these remains belong 

 to a group distinct from both the Cynodontia and Gomphodontia. 



A small badly preserved fragment of a skull found near to this 

 fossil is described, but there appears to be no sufficient evidence for 

 associating it with the other remains. 



XVIII. " On the Evolution of the Vertebral Column of Fishes." 

 By H. GADOW, Ph.D.. F.R.S., and Miss E. C. ABBOTT. 

 Received June 20, 1894. 



(Abstract.) 



Concerning the segmental mesodermal products the following sub- 

 division is adhered to : 



The term myotome is to be restricted to the whole rest of the 

 protovertebra after the skeletogenous cells have been given off for the 

 production of the sklerotomes. 



The sum total of the sklerotomes makes up the skeletogenous 

 layer. 



The ending tome to indicate the primary, or earlier, less differen- 

 tiated ; the ending mere to signify the final condition or product. 



Consequently, the protovertebrae divide into I, Myotomes, each 

 of which produces (1) one myomere or segment of the general mass 

 of trunk-muscles, (2) cutis ; II, Sklerotomes which produce sklero- 

 meres or skeletal trunk segments. 



Each protovertebra produces a dorsal and a ventral sklerotome ; 

 strictly speaking, one sklerotome which consists of a separate dorsal 

 and ventral half 



The protovertebral segments are not transverse " plates," but are 

 curved into S-shape, the top end curving tail- and inwards, the middle 

 and ventral thirds bulging headwards, the amount of curvature being 

 (in 7 mm. embryos of Acanthias) so great that a transverse plane 

 will cut through the dorsal and ventral third of one, and through the 

 middle portion of the next following segment. 



This S-shaped curving and consequent overlapping of the proto- 

 vertebral " plates " SS is of fundamental importance for our under- 





