12 OSTEOLOGY OF PTERANODON. 



No. 538), to show the form and position of the occipital style and the attachment of 

 the temporal muscles, the arrangement of these muscles being adapted from Yarrell's 

 figure. As Yarrell's description of this structure can not be improved upon, it is quoted 

 here : 



"This additional bone (occipital style] is about one inch in length, triangular in shape, 

 somewhat grooved on its surfaces, and from its articulation with the occiput tapers 

 gradually to a point. The mode by which this bone is articulated to the occiput is 

 similar to that observed in the ribs of serpents, in which the condyle is situated upon 

 each vertebra, and the cavity is at the end of the rib ; so in the Corvorant, the con- 

 dyle is upon the occipital bone, the cavity at the triangular end of the xiphoid bone : 

 the joint is therefore hemispherical : admitting great extent of motion, the advantages 

 of which will be hereafter pointed out. 



" From the upper edge of this bone to its lateral angle throughout its whole length 

 from the extreme point to the occiput, there arises on each side a triangular-shaped 

 long muscle, the fibres of which are directed forwards, downwards and outwards to be 

 inserted by a strong tendon upon the upper edge of the lower mandible, immediately 

 behind the insertion of the tendon of the temporal muscle. The muscles of the upper 

 part of the neck, giving motion to the head, are inserted upon the occipital bone and 

 its elevated crest, over which these additional muscles slide with every movement of 

 the head, the particular articulation of the xiphoid bone only permitting it to become 

 a fixed point of support to its own particular muscles, when both act simultaneously 

 as additional elevators of the lower mandible, thus assisting in prehension, and materi- 

 ally increasing the power of the bird in securing a slippery prey." 



The literature on the anatomy of Plotus, the Darter, is equally complete. Professor 

 A. H. Garrod states in his paper : " Notes on the Anatomy of Plotus anhinga " (Proc. Zool. 

 Soc. London, 1876):- 



" In the Society's female specimen there is a fibro-cartilaginous similarly situated 

 process, not more than one sixth of an inch long, which is ossified in the evidently 

 older male. In .his notes on the anatomy of the Cormorant, Hunter tells us that ' a 

 small bone, about an inch long, passes back from the os occipitis and gives origin to 

 the temporal muscle, which is very strong.' The same bone in the Darter, although 

 comparatively not so long, performs the same function, the superficial temporal muscles 

 meeting behind the skull along the median raphe, which becomes ossified to form the 

 above-mentioned bony style in the adult bird." Professor Garrod's drawing is exactly 

 reproduced in Plate V, figure 12, although the length of the occipital style as it appears 

 in the figure does not accord with the length given in the text. 



In a later paper (Proc. Zool. Society, London, 1878), Professor Garrod records that in 

 P. levaillanti, the African Darter, the temporal muscles run back beyond the skull, being 

 separated by a median fibrous raphe, which is not ossified into a separate bony style. 



In the five recent animals here referred to are seen four different stages in the de- 

 velopment of an occipital crest, Chelydra and Chamseleo representing the same stage as 

 far as the present discussion is concerned. 



The first and lowest stage is that ascribed to Plotus levaillanti and to the female of 

 P. anhinga, in which the enlarged temporal muscles meet behind the cranium and their 

 line of contact has been occupied by a minute " fibro-cartilaginous process." In the 

 older male of P. anhinga, the second stage, the fibro-cartilage has become ossified and 

 the process converted into a true bony style. In the third stage, represented by Phala- 



