Being especially strong and massive, isolated diodon- 

 tid jaws and fragments have been widely found in every 

 period from the Eocene onward, and the confused 

 nomenclature given to these parts has been extensively 

 and astutely reviewed by Tavani (1955), who recognized 

 several new genera on the basis of dental structure, as 

 only briefly summarized here. Tavani found that Eodio- 

 don, from the middle and upper Eocene of Europe, dif- 

 fered from all other diodontid jaws by having a uniform 

 bony mass not differentiated into small teeth at the bit- 

 ing edge and larger elements in an internal trituration 



plate, as well as a difference in the place of articulation 

 with the palatine. 



Tavani (1955) recognized Eodiodon as the family 

 Eodiodontidae, in contrast to four genera with distinct 

 and differentially formed teeth in the biting and tritura- 

 tion regions. Of the latter four genera, Tavani differenti- 

 ated Progymnodon (see Dames 1883), from the middle 

 and upper Eocene of Europe, and Kyrtogymnodon, from 

 the Pliocene of Europe, from the remaining two genera, 

 Oligodiodon from the Oligocene and Miocene of Europe 

 and North America, and Diodon, from the Miocene of 

 Europe to Recent, on the basis of the depth of the osse- 

 ous jaw tissue separating the small teeth in the biting 

 edge from the large teeth in the trituration plate. Pro- 

 gymnodon and Kyrtogymnodon, as the subfamily Pro- 

 gymnodontinae, have only a thin band of bony tissue 

 separating the two, and Oligodiodon and Diodon, as the 

 Diodontinae, have a broad band. 



The difference between the Progymnodon and Oligo- 

 diodon type spatial arrangement as so carefully de- 

 scribed by Tavani (1955) undoubtedly represents the 

 evolutionary change that took place leading to the 

 arrangement as found in the Recent species, in which the 

 biting edge and trituration teeth are well separated by a 

 deep layer of bone, although less so in young specimens 

 than adults. However, I am skeptical about the reported 

 dental condition in Eodiodon, of it not having any teeth 

 differentiated from the osseous tissue of the jaws. My 

 skepticism is primarily because in the two groups most 

 basal in the ancestry of the gymnodonts, the eoplectin 

 triacanthodids and triodontids, the teeth are clearly dif- 

 ferentiated from the bone, as they are in all diodontids 

 from the Eocene to Recent, with the possible exception of 

 Eodiodon. 



I suspect that the apparent lack of discrete teeth in 

 Eodiodon alone is some artifact of preservation of the few 

 known jaw fragments assigned to Eodiodon, or that the 

 biting edge teeth and those of the trituration plate were 

 lost sometime between the process of preservation and 

 the eventual recovery of the abraded fragments. With the 

 true nature of the dental units of Eodiodon perhaps still 

 unknown, and with no knowledge of what the structure of 

 the fish behind the jaws was like in Eodiodon, Progym- 

 nodon, Kyrtogymnodon, and Oligodiodon, and only the 

 slightest knowledge of the body of Prodiodon, I feel no 

 compulsion for recognizing Eodiodon as familially dis- 

 tinct from the other diodontids, and none for recogniz- 

 ing subfamilies on the basis alone of the width of the os- 

 seous tissue separating the biting and trituration teeth, 

 while still being keenly aware of the great value of 

 Tavani's work. 



With the exception of Tavani's (1955) careful analysis 

 of the dental structure in fossil diodontid jaws, and of the 

 few hints about the general configuration of the body in 

 Prodiodon, our knowledge of the anatomical diversity of 

 the family rests essentially on that of the Recent species. 

 The diodontids differ externally mainly in the form of 

 the nasal apparatus (two nostrils or one) and spines 

 (erectile or nonerectile). In times past, only two genera of 

 diodontids were usually recognized, Chilomycterus for 



