DEVELOPMENT OF THE SKULL IN THE OSTRICH TEIBE. 121 



At the base of the cartilage there is a thick crescentic convexity (Plate VII. fig. 4, q.), 

 the horns of the crescent looking forwards, and being very large and swollen. The head 

 of the detached mandibular cartilage (Plate VII. fig. 3, ar.) — wholly unossified at this 

 stage — is scooped to receive the convexity of the base of the quadrate ; the anterior bar 

 is very long (partly seen in Plate VII. fig. 3, above s.a.) and terete, gently tapering 

 towards its blunt anterior extremity ; this is Meckel's cartilage. The posterior bar, 

 or internal angular process, is short, thick, and incurved. The mandibular splints are 

 as far advanced as those of the skull ; and that which overlaps the detached proximal 

 piece (Plate VII. figs. 3-6, sq.) is larger than the parietal ; it is lozenge-shaped, the 

 obliquely-placed inferior angle being the largest, and running down the side of the 

 quadrate cartilage. This splint is the true homologue of the squamosal of the Mammal, 

 of the temporo-mastoid of the frog, and of the preopercular of the osseous fish. Outside 

 the detached bar there are three splints, the dentary being distal, the surangular and 

 angular being proximal (Plate VII. fig. 3, d.s.a.a.) : on the inside there are two, the 

 splenial, or distal piece, and the coronoid, or proximal. 



The pterygo-palatine or second prestomal arch is developed after a lower type, and 

 in a more hurried manner, and there is no free descending ray. It has been completely 

 ossified during the embryonic simplicity of the cellular tissue in which it was roughly 

 premodelled. It is not in the Ostrich composed of more than two pieces, the pterygoid 

 behind, and the palatine in front (Plate VII. fig. 4, p.g.-pa.) ; and the proximal part, or 

 pedicel, is not segmented from the posterior end of the palatine ; it is its orbital process. 

 The two bones articulate by an oblique suture, which is not persistent, being lost again 

 in the adult bird. The ends of this horizontal divided bar differ very much ; that which 

 is formed by the palatine becoming a sharp style which interdigitates with the inter- 

 maxillary splints, whilst the connexion of the pterygoid vnth the quadrate cartilage is 

 by a cup-and-ball synovial joint: the cup belongs to the pterygoid, and the ball (a very 

 important part in a descending survey of the vertebrate skull) to the quadrate cartilage. 

 Another synovial joint; of an oval form, and with a flattish sinuous face, appears on the 

 inner side of the pterygoid ; it articulates with a similar facet on the end of the anterior 

 pterygoid process of the basisphenoid (Plate VII. figs. 2 & 4, a.p.). The splints of the 

 pterygo-palatine rod are in a very feeble condition, the zygoma being a mere tendon, and 

 only the jugal and quadrato-jugal (Plate VII. j.q.j-) being developed. Moreover, the 

 position of these splints is very peculiar ; for although in an early state of the " maxil- 

 lary rudiment" the tissue in which they have been formed lay close outside the pterygo- 

 palatine streak, yet the rapid widening of the mouth has caused the removal of the 

 zygoma far external to the palatine (primordial) rod. 



The facial flaps which grow down from the frontal wall of the early embryo are con- 

 nate, but are generally notched, the notch indicating where a slit should be. In the 

 lobes of this essentially double lamina the trabeculae terminate, and they form its pith — 

 just as the pterygo-palatine streak of tissue forms the pith of the maxillary rudiment, 



MDCCCLXVI. T 



