202 Bashford Dean Memorial Volume 



and in immediate prolongation of the mandibular and hyoid arches. In the Arthrodira 

 we cannot find the necessary room for the branchial apparatus behind the joint between 

 the infero'gnathal and the post-sub-orbital. Another very probable possibility is that the 

 Arthrodira had the branchial apparatus built after a principle unknown in recent fishes. 

 Such extraordinarily constructed animals with such an unusual mouth mechanism may 

 also have had their respiratory organs constructed on some equally remarkable plan. 



The drawings given in Text'figures 88 and 89 of the arrangement of gill arches in 

 Dimchthys must not be regarded as a reconstruction. They are only an attempt to show 

 how the gill arches might have been placed in an Arthrodire. We have no direct facts in 

 any way supporting this proposition. 



THE BIOLOGY OF DIHICHTHTS 



The complete body form of Dinichthys is unknown. But, with the help of Coo 

 costeus, which in its structure was very like Dinichthys, we can make a relatively good 

 picture of the animal. Very likely, Dinichthys had a macruriform body, that is, the 

 highest and broadest part of the body was immediately behind the head. The posterior 

 part behind the body carapace became narrower and narrower, finally culminating in a 

 pointed tail. One dorsal fin and the paired pelvic fins were present. It is not excluded that 

 also the paired pectoral fins were developed. But only the tail served as locomotor organ. 



Dinichthys must certainly be regarded as a carnivorous animal. Only a look, tor 

 instance, at Text-figure 81, must convince us that such a perfectly armed mouth belonged 

 to a carnivore. And, being a relatively good swimmer, it could actively pursue its prey. 

 Text-figure 90 shows a tentative total reconstruction of Dinichthys. 



THE RELATIONSHIPS AND SYSTEMATIC POSITION 

 OF THE ARTHRODIRA 

 It is doubtful whether any other group has been assigned relationships to such a 

 number of other animal groups as has been the case with the Arthrodira. At first de- 

 termined as water beetles (reference in Agassiz, 1844, p. xxx), they were later described as 

 fragments of reptiles (Kutorga 1835, 1837). Assmuss (1840) was the first who positively 

 determined them as belonging to the fishes. After his time the Arthrodira were moved 

 from one group of fishes to another. They were placed among the Ganoidei (Agassiz, 

 1833; Eichwald, 1860; Zittel, 1887): together with the Antiarcha were made an in- 

 dependent family of fishes, the Placodermata (M'Coy, 1848.3; Pander, 1857; Huxley, 

 1861; Traquair, 1888; Regan, 1904): were thought to be related to the Chondrostei 

 (Baer, 1840; Jaekel, 1906-1929); to the Dipnoi (Newberry, 1875; Woodward, 1891.1; 

 Dean, 1895; Eastman, 1900-1917); to the Holocephali (Jaekel, 1902); to the Elasmo- 

 branchii (Woodward, 1924; Stensio, 1925); and to the Actinopterygii (Traquair, 1900.1; 

 Bridge, 1904). Dean (1900-1901) and Hussakof (1905-1907) placed them as an independent 

 class, the Arthrognathi, equivalent to the class Pisces. Finally, Jaekel (1906. 1) wrote that : 

 ''Gerade die Organisation der Placodermen spricht deutlich fiir eine Abstammung der Fische 

 von alteren, uns noch unbekannten, vierfiissigen Landwirbeltieren." 



