The Structure of Dmichthys 



201 



tively strongly compacted. It consisted of cartilage. No real movable bony operculum 

 was developed. The respiratory movements either functioned with the help of a special 

 inner musculature or in combination with the lifting and dropping of the head in opening 

 the mouth. 



If the gill arches in the Arthrodira existed conformably with those in the fishes, 

 they must have been placed like the arrange- 

 ment in Teleostomous fishes, but not as in 

 Elasmobranchs. This assertion becomes clear- 

 er if we look at the relative arrangement of 



Text-figure 



The author's reconstruction of the possible arrange- 

 ment of the gill arches in Dmichthys, in lateral view. 

 To show the gill arches (G), PSO, hind part of SO, PIG, and hind 

 part of IG have been left off. 



Text-figure 89. 

 A reconstruction of the possible arrangement of the 



gill arches in Dinichthys seen from below. 

 To show the gill arches (G), the musculus depressor gnathalis 

 (IV) on the left side has been omitted. 



the infero-gnathals (IG) and the front part of the ventral shield in the Arthrodira, and 

 compare them with the corresponding parts in present-day fishes. 



This arrangement reminds us very much of that in the higher fishes. The intero- 

 lateral, together with parts of the antero-lateral, corresponds with the clavicle; the 

 infero-gnathal with the mandibular arches. The system of five or more gill arches can be 

 placed between the infero-gnathal and the intero-lateral in accordance with the arrange- 

 ment in the higher Teleostomi. The inner wings of AL and IL form the hind wall of the 

 gill chambers. Along the median line, from AM V to the symphysis of the infero-gnathal, 

 the basi-hyal elements may be placed. We know nothing about the presence or absence 

 of hyoid arches. Their reconstruction as given by Jaekel (1919-1925) is very problematic. 

 If the inferno-gnathal is only a superficial bone, not corresponding to the lower jaw in 

 other vertebrates, we can not expect the Arthrodira to have developed hyoid arches. 



To reconstruct the gill apparatus in Dinichthys in accordance with that of the 

 Elasmobranchs is still more difficult. In all these forms the gill arches are placed behind 



