LIASSIC FORMATIONS. 



57 



previously known Pterosauria, and adding such deductions as to the status and allinitics 

 of the order as seem legitimately to flow from the facts. 



The first distinguishing feature of Bimorphodon, or of the present liassic type of the 

 genus, is the disproportionate magnitude of the head — the more strangely dispro- 

 portionate, as it seems, in an animal of flight. 



The head is large in proportion to the trunk, not only in respect of length but of 

 depth, and probably, also, breadth ; nevertheless, the shape and disposition of the con- 

 stituent bones are such that, perhaps, no other known skull of a vertebrate is constructed 

 with more economy of material — with an arrangement and connection of bones more 

 completely adapted to combine lightness with strength. 



So far as the skulls of Pterosauria have been sufficiently entire to show the shape of 

 the head, no other known species resembles Bimorphodon. The cranial part is singularly 

 small : the rest is mainly devoted to the formation of the large, long, and powerful 

 prehensile and manducatory jaws. Among the debris of the cranial bones, in specimens 

 Pis. XVII and XVIII, the mastoid (s), parts of the occipital (paroccipital, 4), the parietal (7), 

 post-frontal (12), frontal (11), prefrontal (14), and nasal (15), are recognisable : the last two 

 bones, however, are concerned more with the scaffolding or buttressing of the upper jaw 

 than with the protection of the brain or formation of its case. Though contributing their 

 shares to the otccrane, the chief developments of the paroccipital (PI. XX, 4) and mastoid 

 (ib. 8) relate to the nniscular connections of the head with the trunk : the mastoid joins the 

 postfrontal to form an upper zygoma, giving origin to part of the temporal muscles ; it also 

 affords a fixed articulation to the tympanic, and sends down a pointed process external to the 

 masto- tympanic articulation. The parietals (PI. XX, 7 ), confluent at the mid line, where they 

 develop a low crest, swell out slightly at the temporal fossa, indicative of the size and saurian 

 position of the mesencephalon. The frontal (11) is narrow and flat between the orbits, of 

 which it contributes most of the upper part of the rim. This is continued by the postfrontal 

 (12) behind, which sends down a long pointed process to unite with the malar (26), and a 

 shorter and thicker one to join the mastoid (s). The prefrontal (14), of a triangular form, 

 contributes to the upper and fore part of the orbit, and, either directly or by a connate 

 lacrymal, unites with the ascending malo-maxillary process (21, 2G), and the base of the pre- 

 frontal articulates with the frontal and the nasal. The nasals (15), to the usual con- 

 nections with the frontal, prefrontal, and medial process of the premaxillary (22'), superadd 

 a union with the lateral ascending process of the maxillary (21^), completing the bar 

 between the nostril {n) and the antorbital vacuity («). The nasal bone forms the upper part 

 of the nostril ; the rest of the boundary of that singularly wide aperture is formed by the 

 premaxillary and maxillary. Of the basis cranii and palate there do not appear to be 

 any recognisable parts preserved. The maxillary is overlapped by the hind alveolar part 

 of the premaxillary, and unites therewith by a long oblique suture (21")- I'he maxil- 

 lary, receding, expands and sends upward a long slender pointed process to articulate 

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