- 



PLEONASTE, 



PLESIOSAURUa 



Uil. surmounted with crests, as in the Crocodile* ; the 

 other with the Uil perfectly conical, or Terr (lightly flattened on iu 

 four aurCtoea, w that it itill appears rounded. 

 I'LKoNASTK. (Srsxu.1 



PLESIOSAU'RUS (Couybeare), a genus of the extinct family of 

 Eoaliosaiiriana. Thii genui wat Ant described by Mr. Conybeare, 

 with the uaistance of Sir Henry de la Beche, in the ' Transactions of 

 th* Geological Society of London,' in 1821. A restoration of the 

 skeleton, together with that of /dUAyoMunu, was given by Mr. Cony- 

 bear* in a Mibeequent volume of the aame ' Transactions ' (1824), from 

 an almost perfect specimen of I'Uttcaaurtu doiichodeinu, and the 

 accuracy of thu restoration is still universally acknowledged. This 

 gnu* is the Ilolulraca* of Wagler. 



The 'tirtingimhing features of the Enaliosauriani consist in the 

 abecoce of the ball and socket articulations of the bodies of the verte- 

 bra, the position of the nostrils at or near the summit of the head ; 

 their separated bjemapopbyses ; the numerous short and flat digital 

 bunts, which must have been enveloped in a simple undivided tegu- 

 inenUry sheath, forming in both the fore and hind extremities a fin 

 resembling in external appearance the paddles of the Cetacea. 



The characters of the genera PUnotattnu and Ichthyotauriu, the 

 types of the two principal modifications of anatomical structure 

 offered by the Enaliosauriani, are mainly derived from modifications 

 of the vertebral column, as well with regard to the form and configu- 

 ration of the individual bones, as to the relative groups of the cervical, 

 dorsal, and caudal vertebra. [ICHTBTOCAUBUS.] Professor Owen has 

 also found that the vertebra) afford the best characters for the distinc- 

 tion of species as well as of genera. 



The most conspicuous and remarkable character of the vertebra) in 

 TtesMMnnu U the extraordinary length of the cervical portion, 

 including from twenty to forty vertebra. The articular surfaces of 

 their bodies are either flat or slightly convex in the centre, and most 

 frequently concave at the periphery. Two piU are generally presented 

 at the under part of the bodies, but this character U not constant. 

 The cervical vertebra of the Plerioiauri generally present the centrum, 

 the neurapophyses, and the ribs in a separate or unanchylosed state, 

 and although in general no transverse processes are developed in this 

 region, an analogy with the structure characteristic of this part of the 

 (pine in the Crocodile is maintained in the division of the articular 

 surface for the cervical rib into an upper and lower portion by a 

 horizontal fiisure a structure which, Professor Owen observes, is well 

 described and figured by Conybeare in the Plesiotauriu dolic/todeiriu : 

 as these vertebra of the neck approach the dorsal, the inferior part 

 of the costal articulation becomes smaller, and a corresponding 

 increase of surface is afforded by the superior facet, which also 

 gradually rises from the centrum to the neurapophyses, and in the 

 ilonal vertebra stands boldly out as a true transverse process from 

 the upper side of the base of each neurapophysis. The transverse 

 prooeesM of the sacral vertebra subside to the level of the neura- 

 pophyses; and as the caudal vertebra recede from the trunk, the 

 articular surface, which, as in the neck, represents or takes the place 

 of the transverse process, gradually descends, and passes from the 

 neurapophyses to the side of the centrum, but in not divided by the 

 longitudinal groove which characterises the costal surface in the neck ; 

 but it must be borne in mind that this groove is more marked in some 

 species of Plaiotaunu than others ; and indeed Professor Owen has 

 seen Pleeioaaurian cervical vertebra in which no trace of it was visible. 

 " The neurapophysei," he says, "are commonly unanchylosed with 

 the vertebral centres hi any part of the spine, and in some instances 

 throughout the cervical and at the anterior part of the dorsal region 

 the neurapopbysea have appeared to be joined each by an articular 

 surface to the spine above, as they are to the centrum below the 

 pine* here remaining apparently throughout life unanchylosed to 

 the neurapopbyses. This condition of the upper vertebral element* 

 U rarely seen in any cold-blooded vertebrate animals, and never in 

 the warm-blooded classes. In those part* of the spine where the 

 vertebra enjoyed leu mobility upon each other than in the neck, the 

 H>ian become ancbylosed to the neurapophyses at an earlier period. 

 The hxmapoptiyses co-exist with the ribs or paravertebral elements in 

 the caudal region of the spine, but they continue throughout life to 

 bo unattached by bone either to the centrum above or to each other 

 b* low ; and here also their spine u not developed, and consequently 

 no true chevron bone is formed in the PUiiomiri. The hiemapophyses 

 are also continued along the inferior surface of a great part of the 

 abdomen, forming there the sternal or abdominal ribs ; and just as the 

 neurspophy.es are developed in the transverse direction to protect 

 the expanded cerebral mssscs hi the cranial region, so here the hroma- 

 popbyses are in like manner elongated transversely, and their spine 

 i introduced and modified to form a third mesial rib-like bar, con- 

 necting however, as usual, the lower or distal extremities of the 

 banaapopbyses, and completing the oeaeous cincture of the abdominal 

 viscera. The tail in the Pletiotauri is relatively much shorter than in 

 the /cAMyoKmri, and there is an obvious reason for the curtailment of 

 this part of the animal ; for in the Pleiiotauri, the length and mobility 

 of the neck renders a special development of the Uil for producing 

 tbe lateral movement* of the head unnecessary. The bodies of the 

 vvrUbm, in most species of Plt*%a*anu, are traversed vertically by 

 two vascular canal., which lead from the medullary or spinal canal 



to the inferior surface of the centrum, where they terminate each by 

 an orifice, and sometimes by two orifices on each side of the middle 

 line. These orifices are not however a conrtaut character of the 

 genus Plaiotaurut, neither are they peculiar to this genus, being 

 present in the vertebra of the Cetaeea, as well u in those of other 

 Sauna. In a section of the vertebral centrum of a PUtionuriu, the 

 osseous texture for some lines near the anterior and posterior articular 

 surf ices is denser than in the rest of the vertabrae, and the direction 

 of the lamina) and fibres U vertical ; in the intermediate portion the 

 lamina) are horizontal." 



Though the head of the Plaiotauri resembles that of the crocodiles 

 in its general form, it is, relatively much smaller in proportion to the 

 body. The elongated form of the strong and prominent cranial bones, 

 most of which extend from point to point, with wide interspaces like 

 the timbers of a scaffolding, forms one of the first indications of a 

 deviation from the Crocodilian type, and of the affinity of the Plaio- 

 laurta to the Lacertian Sauria ; and this affinity is further exemplified 

 in the condition of many of the individual bones. 



The parietal bone is strong and triradiate, consisting of a median 

 piece corresponding with the normal parietal in the Crocodiles, and of 

 two transverse elongated processes, formed, as it were, by a bifurcation 

 of the posterior part of the median piece ; and in a young specimen 

 of PUtioiaurut macroeephalui, in the museum of the Earl of Ennis- 

 killen, Professor Owen found the median or sagittal suture dividing 

 the two parieUls still distinct ; in older specimens of 1'. JIawkinrii he 

 always found it obliterated ; affording evidence to justify the descrip- 

 tion of the parietal bone as single or triradiate. The other bones of 

 the head present many remarkable distinctions from the ordinary 

 Crocodilean type. The general structure of the head will be seen in 

 the accompanying cuts of the skull of the PUriotauriu restored after 

 Conybeare. 



Skull of ririioiaurut restored : a, profile ; 6, Hern from above. 



Vnikr jiw of I'/itiotaxrui (anterior part). 

 a, icon from above ; , profile; e, teen from below. (Cony wart.) 



The usual complicated structure observable in the Saurian* appears 

 in the Lower Jaw of the Pietioiaurui. The dentary piece presents 



