Fin rays and ptery^ophores. — Twenty-six fin rays 

 are present in most of the study specimens; the first ray 

 unbranched, the others branched in single or double 

 dichotomies. Each fin ray has a small, unpaired, distal 

 pterygiophore between the bifurcate base of the ray. 

 Basally the fin rays are supported by 27 basal pterygio- 

 phores. Each basal pterygiophore has a well-developed 

 lateral flange throughout its length, except at the very 

 proximal and distal ends of the element, for muscle at- 

 tachment. The lateral flanges decrease slightly in height 

 posteriorly in the series. The first basal pterygiophore is 

 somewhat shorter, but stouter, than those just posterior 

 to it, and articulates by fibrous tissue ventrally between 

 the neural spines of the fifth and sixth abdominal verte- 

 brae, while anteroventrally it supports the base of the 

 supraneural. Posterodorsally the first basal pterygio- 

 phore interdigitates with the second basal pterygio- 

 phore, and all of the subsequent basal pterygiophores ex- 

 tensively interdigitate to one another along their edges of 

 contact, the more posterior elements only slightly less ex- 

 tensively so than more interiorly. The last few basal 

 pterygiophores articulate between the neural spines of 

 the seventh and eighth caudal vertebrae. The degree of 

 interdigitation between the pterygiophores, and their 

 closeness of apposition with the neural spines, increases 

 with increased size of the individual, so that in very large 

 specimens a massive, almost solid plate is present 

 between the vertebrae and the soft dorsal fin. The basal 

 pterygial elements are cartilage filled at their dorsal and 

 ventral edges. In all but one of the five study specimens 

 the number and arrangement of the basal pterygial 

 elements is as described above, but slight variation in the 

 number of elements is to be expected. 



Fin rays and pterygiophores. — Twenty-three fin 

 rays are present in most of the study specimens; first ray 

 unbranched, the others branched in single or double 

 dichotomies. Each fin ray with a small, unpaired, distal 

 pterygiophore between its bifurcate base. Basally the 

 rays are supported by 22 basal pterygiophores, like those 

 of the dorsal fin. The first basal pterygiophore is by far 

 the largest of the series and has its anterior edge ex- 

 panded laterally. It articulates by fibrous tissue dorsally 

 between the haemal spines of the first and second caudal 

 vertebrae, while posteroventrally it interdigitates with 

 the second basal pterygiophore, and all of the sub- 

 sequent basal pterygiophores are extensively inter- 

 digitated to one another, as in the soft dorsal fin, with the 

 interdigitation and closeness of apposition to the haemal 

 spines increasing with increasing specimen size. The last 

 few basal pterygiophores articulate between the haemal 

 spines of the seventh and eighth caudal vertebrae. 



Anatomical diversity.— The balistids are one of the 

 least anatomically and otherwise diversified families of 

 plectognaths, of a degree of diversity comparable to that 

 of the Triacanthidae, Aracanidae, and Diodontidae. 

 Internally the balistids differ remarkably little from one 



another in fundamental plan. The vertebral column (7 -(- 

 11), basal pterygiophore support system of the soft dor- 

 sal and anal fins, and the caudal fin supporting struc- 

 tures are basically the same in all species. The skulls 

 differ mainly in the degree of development and massive- 

 ness of the supraoccipital and epiotic supports of the 

 basal pterygiophores of the spiny dorsal fin (least 

 developed in Rhinecanthus), perhaps correlated at least 

 in part with the size of the third spine, and of the degree 

 of anterior displacement of the suspensorium (most dis- 

 placed in Xanthichthys and, especially, Odonus), cor- 

 related with the degree of upturning of the mouth. 



The genera of balistids are, in fact, distinguishable 

 mainly by external features such as: the degree of 

 development of a cusp or enlargement on the medial end 

 of the biting edge of the teeth (least developed in Melich- 

 thys, in which at least the more medial of the cusped 

 teeth of young specimens become truncate true incisors 

 with increasing size; best developed in Xanthichthys 

 and, especially, Odonus, in both of which the most 

 medial tooth of the premaxillary is smaller than that just 

 lateral to it, whereas it is the medial tooth that is the 

 largest in all other balistids; the enlarged cusp of the sec- 

 ond most medial tooth in Odonus projecting far beyond 

 the others and resting on the lower lip as a fang when the 

 jaws are closed); the presence or absence of a deep groove 

 in front of the eye and below the nostrils of unknown 

 functional significance (absent in Balistapus and Rhine- 

 canthus, and only weakly present in Pseudobalistes); the 

 presence or absence of enlarged, rounded, or otherwise 

 modified nonoverlapping and flexible scales covering the 

 tympanum (not enlarged in Canthidermis and 

 Xanthichthys); scales and spinulation patterns on the 

 head and body; degree of development of the third dor- 

 sal spine (least developed in Melichthys, Rhinecanthus, 

 and Xanthichthys; becoming minute in large specimens 

 of Canthidermis and Balistoides conspicillum) , these 

 diagnostic features supplemented by the shape of the 

 caudal peduncle and caudal fin. These and other features 

 in the anatomical diversity of the Recent species are dis- 

 cussed further in the section on generic relationships. 



The balistids have changed little since the Oligocene. 

 In the black schist of Canton Glarus, Switzerland, there 

 are three species of balistids placed in the genus Balisto- 

 morphus that are, from what little is known of them from 

 their impressions, thoroughly modem in appearance. 

 The holotypes and a few additional specimens of these 

 three species have recently been reexamined and data 

 on them is given below. 



Balistomorphus ovalis (Agassiz 1842 illustration, 1844b 

 description) is the most slender bodied of the three 

 species in the genus. The holotype, IGUN uncatalogued 

 (but XXXVH scratched on the back of the plate), a 

 single impression (head left) is 121 mm SL. The depth of 

 the body between the soft dorsal and anal fin origins is 

 45.3 mm (37.3% SL). The least depth of the caudal 

 peduncle is 12.8 mm (10.5% SL). The length of the first 

 dorsal spine is 25.7 mm (21.2% SL). The second dorsal 

 spine cannot be measured, but the third spine is 4.5 mm 

 (3.7% SL). There are 11 caudal vertebrae, as in Recent 



