94 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



(4) That the nasals of teleosts have come from the nasals of eugnathids and semionotids, 

 which lie above the narial opening in front of the frontals. In the palaeoniscoids the nasals 

 (of teleosts) may possibly be represented by the bones called by Watson postrostrals. 

 Perhaps eventually the nasals may be traced to three sets of paired lateral "nasals" in the 

 primitive rhipidists. According to this interpretation of the facts the large size of the 

 median ethmoid ("mesethmoid") and the small size of the nasals in typical teleosts reverse 

 the conditions in their protospondyl predecessors; the teleosts have also lost most of the 

 rostral mosaic, retaining only the median rostral in the form of a more or less enlarged 

 "mesethmoid." 



The mesethmoid of teleosts has been shown by Starks to vary enormously in the differ- 

 ent groups. "The scale-like, thin, superficial form of mesethmoid" (writes Starks, 1926a, 

 p. 326), "that overlies the ethmoid cartilage and is doubtless of purely dermal origin, surely 

 must be the most primitive, but it does not always co-ordinate with other primitive char- 

 acters. Some of the primitive fishes, as some of the plectospondyli, seem to have as complex 

 and highly developed a mesethmoid as do the highly specialized and advanced spiny-rayed 

 fishes. The simplest form of it seems to appear first among the Salmonidce , and especially 

 among the Argentinidce, though in the latter its exceedingly filmy condition possibly indi- 

 cates some degree of degeneration. On the other hand, the disk form of mesethmoid is 

 found in more advanced families, as the AtherinidcB and Poeciliida, or such highly specialized 

 forms as some of the Labrididcs. In many of the higher forms, such as the last-mentioned 

 family, its modification to the disk-like form seems to have been brought about by the 

 development of the premaxillary processes, that lie over it and depress it." 



In the higher or spiny-rayed fishes the mesethmoid is usually of dual origin, its surface 

 being ectosteal and its interior endosteal (Starks, 1926a, p. 327). The detailed changes of 

 this element in the different groups have been described by Starks. 



As Table I has to do chiefly with the normal elements of the typical fish skull, it does 

 not include the following occasional elements of the ethmoid region, as described by Starks 

 (1926fl, pp. 332-335): 



Pre-ethmoids. — Paired lateral ossifications in Amia and Esox, lying just above the 

 vomer. The palatines are attached or closely connected with them. Somewhat similar 

 elements in the Eventognathi (suckers and minnows) may be homologous with the pre- 

 ethmoids, or may have arisen as epiphyses of the more or less ossified submaxillary rods 

 (see below). 



Submaxillaries. — "Sagemehl (1891) has applied this name to a pair of rods that con- 

 nect the maxillaries with the pre-ethmoids in Catostomus. They may be cartilaginous or 

 partly ossified or wholly ossified. In the carp-like fishes they are reduced to double concave 

 disks of fibro-cartilage. In most other fishes a thin, fibrous pad under each maxillary rests 

 on the vomer and is apparently homologous with the submaxillary. Occasionally the pads 

 may ossify as described under Sardinia and other clupeoid fishes" (Starks, 1926a, p. 336). 

 In other words, these elements appear to be like sesamoid or tendon bones of mammals. 



Proethmoids oj Esox. — Paired bones resting on the frontals behind and on the cartilage 

 above the vomer in front. Present in some other haplomous fishes and possibly combined 

 with the nasals in Tylosurus. Probably not equivalent with the mesethmoid (Starks). 



Rhinos phenoid. — A median bone of unknown origin, forming a septum between the 

 olfactory nerves as they issue from the orbitosphenoid. Known only in two characinoid 

 fishes (Starks). 



