Structure of the Plesiosaurus Macrocephalus. 533 



separated by a flattened interspace of the parietal bone. The Plesiosaurs 

 present a still more marked affinity with the Squammate Saurians, and devia- 

 tion from the Loricate tribe, in the posterior bifurcation and median foramen 

 of the OS parietale. 



The general condition of the present fossil indicates it to belong to a young 

 individual, and to this circumstance may be attributed the non-consolidation 

 of the two parietals into a single median bone, as is its condition in all other 

 Saurians. 



The median frontals extend forward as faras the midspace between the small 

 nostrils, and appear in the PL Macrocephalus to terminate in a point between 

 the commencement of the narrow nasal bones. The interfrontal suture is 

 elevated by a ridge continued forwards from that of the interparietal. The 

 outer margin of the median frontal forms the superior boundary of the orbit. 

 The anterior frontal enters into the formation of the anterior and superior 

 angle of the orbit, and is wedged in between the mid-frontal and superior 

 maxillary bones. The posterior frontal, which is here narrower than in the 

 PL Hawkinsii, resembles the columnar portion of the corresponding bone in 

 the Crocodile, which bounds the orbit posteriorly and extends downwards to 

 join the malar bone ; but this bone is more superficially situated in the 

 Plesiosaurs, which thus resemble more the Lacertine Saurians.. The affinity 

 of the Plesiosaurs to this group and their recedence from the Crocodilian 

 type is more strikingly evinced in the non-extension of the posterior frontal 

 to join the mastoid, and the consequent absence of the osseous ridge which 

 traverses longitudinally the temporal fossa like a second zygoma, and pecu- 

 liarly characterizes the cranium of the Loricated Saurians. 



The zygomatic portion of the temporal also resembles the Lacertine and 

 deviates from the Crocodilian type in its elongated form and horizontal longi- 

 tudinal position. The inner surface of this bone is seen on the leftside of the 

 head, while only that part which joins the malar and posterior frontal remains 

 on the right side in the present specimen. The tympanic bone, in its general 

 form and especially its length, is intermediate to the Crocodilian and Lacer- 

 tine types ; while in regard to its strength it exceeds that of all existing Sau- 

 rians. The expanded inferior termination of this important bone, which gives 

 the surface of articulation to the lower jaw, appears in the present specimen 

 to be formed by a distinct osseous piece or epiphysis, but the difficulty of dis- 

 criminating between suture and fracture in a crushed fossil leaves me in some 

 uncertainty regarding this point : I may observe, however, that a supplement- 

 ary bone, but of much smaller size, occupies a corresponding situation in the 

 cranium of the Monitor. 



3z3 



