48 ILLINOIS BIOLOGICAL MONOGRAPHS [48 



semicircular canal. The canal component, apparently always united with the 

 underlying membrane bone, may as a so-called dermal bone, be found fused 

 with other, adjoining dermal bones; while the primary ossifications may be 

 fused with other adjoining primary ossifications (Ctenodus), or with such 

 ossifications and the intercalar (Polypterus). It is the primary part of the 

 bone, and not its 'Deckenknochenanteil' that gives articulation to the 

 hyomandibular. " 



Throughout the discussion he does not give any reason for calling this bone 

 the squamosal. The term squamosal signifies a homology with the squamosal 

 of the Tetrapoda, which only a part of this compound bone of the fishes has. 

 According to the researches of Thyng (1905) on the squamosal bone of the 

 Tetrapoda, some of the criteria for the establishment of its identity throughout 

 the series were as follows: "The term squamosal, including its various forms, 

 was first applied to an element in the human skull which later fuses with others 

 to form the temporal bone Hence in applying the term to the lower groups 

 the laws of nomenclature demand that it be given to that element which is the 

 homologue of the squamosal in man. It is also evident that all possible criteria 

 should be utilized in settling these homologies, not alone those of adult relation- 

 ships and articulations, but those of development as well. . . 



"The development of the mammalian squamosal shows it to be a membrane 

 bone which overlies the otic capsule and is at first intimately connected with 

 the incus (quadrate) by a dense and fibrous stroma. Hence it must be con- 

 cluded that close association with the quadrate (incus) and the otic capsule is 

 the primitive relation of the squamosal, and therefore, the most important 

 criterion in ascertaining its homology in the non-mammalian vertebrates. " 



Comparing the above with the statements of Allis, it is clear that the only 

 bone in the fishes which may be compared to the mam.malian squamosal is 

 that dermal ossification which overHes the otic capsule above the lateral semi- 

 circular canal. The question of connexion with the underlying primary ossifica- 

 tion is secondary, as is also that of the quadrate, for in the fishes the quadrate 

 is separated from the cranimn by the hyomandibular. The true squamosal 

 would therefore, be the dermal ossification, described by Allis as lying above 

 the cartilage of the wall of the lateral semicircular canal; it has nothing to do 

 with the articulation of the hyomandibular and is, in the fishes, connected with 

 the lateral line osseous canal. The primary ossification underlying it is the 

 pterotic ossification of Parker (1873) and is just as much a center of ossifica- 

 tion of the otic capsule as are the other periotic ossification centers of the mam- 

 malian petrosal portion of the temporal bone, although no center has been 

 found corresponding in position to the pterotic center of the otic capsule of the 

 fishes. 



Schleip (1903) gives a detailed description of the ossifications in the region 

 of the lateral semicircular canal wall in Salmo and follows Gaupp in his nomen- 

 clature, naming parts derived from membrane and tlie lateral line ossification, 



