Mehriam.] 



Triassic Ichthyopterygia . 



101 



side. Excepting these blade-like edges, the cross-section of 

 the shaft of the spine is almost circular near the summit. 

 The height of the spine is relatively greater than in the other 

 forms. As indicated in the table of measurments, this dimen- 

 sion is relatively greater than in vertebra^ from a portion 

 of the column of osmonti slightly anterior to that from which 

 these came, and also greater than that of the late dorsals 

 of pacificus. The zygapophyses are not well preserved, but 

 enough is present to show that they are rather slender and 

 that the right and left facets are not united. The pedicels of the 

 arches show no lateral projection. The general form of the 

 arches present is somewhat similar to that of the eervicals of 

 alexandne. They are, however, too large for eervicals at all 

 comparable to those of other species as regards the relation 

 of their dimensions to those of the middle dorsals. The 

 pedicels are also narrower than in the eervicals. Should it ever 

 appear that cervical arches are here mingled with dorsal vertebrae, 

 this species would be even more sharply separated from the others 

 than it is now held to be. 



The ribs (figs. 5 and 6) associated with the vertebra? just 

 described were most of them lying in such a position as to permit 

 of little doubt as to their having been practically undisturbed. 

 On the broad heads a marked constriction of the middle portion 

 of the articular surface is produced by profound excavation of 

 the posterior side. The upper portion of this surface is consid- 

 erably smaller than the lower and its long axis inclines backward 

 and away from it. The surface also bends sharply outward, 

 deviating as much as thirty-five degrees from the plaue of the 

 lower face The shaft of the rib is almost triangular in 

 cross-section. The upper surface is slightly convex, while 

 the anterior and posterior sides are grooved. Such ribs as 

 those that have just been described are ordinarily asso- 

 ciated with vertebra? in which at least the superior portion of 

 the articular surface rests against the upper arch. There seems 

 to be no question that there is represented here a peculiar mode 

 of attachment , in which the loss of the lower rib head is compen- 

 sated for in the manner described above (page 73). 



