650 S. W. WILLISTON 



to the order, dividing it into the two faniihes Mesosauridae 

 and Paleohatteridae, which he proposed, with the following 

 definition: "Humerus with entepicondylar foramen; five distinct 

 tarsal bones in second row, one for each metatarsal; pubis and 

 ischium broad plates; each set of abdominal ossicles consisting 

 of numerous pieces; condyles of limbs not ossified;" all of which 

 will be found among the characters given for the American Thero- 

 morpha on the preceding pages. It was doubtless under this 

 wider conception of the group, that Broom added to the order, 

 hesitatingly, the genera Saurosternon, Heleosaurus and Heleo- 

 philus from the middle or upper Permian of Africa. These are 

 rejected by Huene,^ and I agree with him. In 1892 Seeley pro- 

 posed the ordinal name Mesosauria to include not only Mesosaurus 

 and Stereosternum, but also some of the Nothosauria.io Osborn 

 in 1904,11 restricted the term to its original limits and his example 

 was followed by McGregor. 



It is very clear that the name Mesosauria, so often used for 

 the group, has no legitimate standing. Seeley and not a few 

 others have believed that the Proganosauria are primitive Sauro- 

 pterygians. Osborn and McGregor, on the other hand, would 

 locate the group among the Diapsida and Diaptosauria, under 

 the assumption that there are two temporal arches on each side, 

 an assumption that has recently been denied by Huene (1. c), who 

 believes that the group is an independent offshoot from the 

 primitive Cotylosauria, and more or less related to the Ichthyo- 

 sauria. He says definitely that Mesosaurus has the supratem- 

 poral fenestra only, and that the lower arch is lost; as Jaekel 

 declares is also the case with the Sauropterygia. There may 

 have been and doubtless were, different ways in which the tem- 

 poral fenestrations arose among Reptilia, but, with all due re- 

 spect to the expressed opinions, I am not yet convinced that the 

 explanations explain. I can conceive how a 'lateral' vacuity 

 may have been converted into an 'upper' one or vice versa, or 

 how the two vacuities may have fused into one, and I am not 



» Paleontographica, vol. 59, p. 100, 1912. 

 10 Quart. Jour. Geol. Soc, 58. p. 586, 1892. 

 'I Mpm. Amer. Mus. Nat. Hist., 1. p. 481. 



