ox CrKTAIX CROSSOPTERYGIANS, 1269 



forms, on the other hand, there nre found merely posterior parts 

 of the basisphenoid and alisphenoid components, while the 

 anterior parts of these components, together with the orbito- 

 sphenoid component, have been reduced and replaced by 

 cartilage. 



In Jlacroponia the part present of the alisphenoid component 

 of either side is continuous with the basisphenoid component, thus 

 having tliere its original relation to the latter component (Watson, 

 1921, p. 323). In Axelia and Wimmiia, on the other hand, the 

 alisphenoid component of each side has coalesced with and forms 

 a descending process from the fronto-dermosphenotic of its side. 

 These different conditions of the alisphenoid component in dif- 

 ferent forms are of very great interest, as they show that a part 

 of a substitution bone may rather easily become independent and 

 coalesce with other neighbouring bones, even dermal ones {cf. 

 Stensio, 1921, pp. 153-161, 181182; Allis, 1899, 1909). 



Bibliography. 



Allex, W. F. 1905. The Blood-vascular System of the Loricati, 



the mail-cheeked fishes. Wash. Acad, of Sci. Proc. 



vol. vii. 

 Allis, E. Ph. 1889. The Anatomy and Development of the 



Lateral Line S\stem in Aviia calva. Journ. of Morph. 



vol. ii. 

 . 1897. The Cranial Muscles and Cranial and First Spinal 



Nerves in Amia calva. Journ of Morph. vol. xii. 

 . 1899. On Certain I-lomologues of the Squamosal, Liter- 



calar, Exoccipital, and Extiasca.pular Bones of Amia 



calva. Anat. Anz., Bd. xvi. 

 . 1900. The Lateral Sensory Canals of Polyjjterus hichir. 



Anat. Anz., Bd. xvii. 

 . 1902. The Lateral Sensory Canals, the Eye-muscles, and 



the Peripheral Distribution of certain of the Cranial 



Nerves of Mu&telus Icevis. Quart. Journ. Micr. Sci. 



vol. xlv, 

 • . 1903. The Skull, and the Cranial and First Spinal 



Muscles and Nei-ves in Scomber scomber. Journ. of 



Morph. vol. xviii. 

 . 1905. The Latero-sensory Canals and Related Bones in 



Fishes. Intern. Monatsschrift f. Anat. u. Physiol., 



Bd. xxi. 

 . 1908. The Pseudobranchial and Carotid Arteries in 



Amevurics. Anat. Anz., Bd. xxxiii. 

 . 1909. The Cranial Anatomy of the Mail-cheeked Fishes. 



Zoologica, vol. xxii. 

 . 1911a. The Pseudobranchial and Carotid Arteries in 



Polyodon si^athida. Anat. Anz., Bd. xxxix. 

 . 1911 b. The Pseudobranchial and Carotid Arteries in 



Chlamydoselachus anguinetcs. Anat. Anz., Bd. xxxix. 

 Proc. Zool. Soc— 1922, No. LXXXJV. 84 



