1896.] MB. G. A. BOULBFGEB ON SCHLESEIi's GATIAL. 633 



the second is deeply bifurcate, the tuberculutn ligamentous, and 

 attached to two processes on the centrum of the atlas. 



Tomistoma has been noticed above. Osteoloemus, curiously, agrees 

 with it. 



We thus see that Metriorhynehus represents the most gene- 

 ralized condition, and that the recent Crocodilians, each depart- 

 ing in its way from the primitive type, cannot be arranged in a 

 continuous series in this any more than in several other parts of 

 their structure. Whilst more generalized in respect to the second 

 rib ' than the true Crocodiles, the Alligator is more specialized in 

 the more aberrant position of the first rib ; the Gavial agrees 

 with the Crocodile in the position of the first rib, and with the 

 Alligator and Caiman in the strong bifurcation of the second ; and 

 Tomistoma and Osteolcemits present the highest specialization in 

 the condition of the second rib with rudimentary tuberculum. 



P.S. (June 18, ISQeV — Two days after the reading of my paper, 

 I received Dr. Gadow's memoir on the Vertebral Column of 

 Amphibia and Amniota (Phil. Trans, clxxxvii. B. pp. 1-57). In 

 this he gives an account and a diagrammatic figure of the atlas 

 and axis of Metriorhynehus, which dift'er entirely from what I have 

 observed. I at once re-examined the 8i)ecimens, and particularly 

 that described by Hulke and figured by Pr. Gadow, and find 



Kg. 2. 



Atlns and axis of Metriorhynchiis. 

 c. Centrum. 1 na. Neural arch. 



cp. Capitular facet. t. Tubercular facet (diapophysis). 



ha. Hjpapophjsis. | 



the latter's statement to be erroneous. What is figured as the first 

 centrum is a portion of the first neural arch, the posterior portion 

 of which has passed, on the figure, into the second vertebra ; the 

 tubercle (<'), to which allusion is made, is on the neural arch. I 

 append (fig. 2) a corrected sketch of the specimen figured by 

 Dr. Gadow. 



' Another character in which Alligator is more generalized than Caiman 

 and Crocodilvs exists in the proatlas, the arches of which are distinct or show 

 at least a trace of separation, which is not to be found in the other genera, even 

 in quite young specimens. 



Pboc. Zool. Soo.— 1896, No. XLI. 41 



