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Bashford Dean Meynorial Volume 



other fishes are unknown). On the other hand, the deep impression between the dorsal 

 and ventral armor can also be explained as the place for pectoral fins. 



In reality, the deep narrow cleft between the dorsal and ventral carapace in Dmich- 

 thys (Text-figure 79) is very remarkable, and hard to explain if we do not accept the 

 presence of fins. A concretion in the American Museum, containing strongly crushed 



fragments of an Arthrodire (not a Dy 

 mchthys), carries on the side of the plate 

 AVL some traces of fin rays, arranged 

 in a parallel fashion (Text'figure 86). 

 This specimen, however, is too badly 

 preserved to make certain that we have 

 here real pectoral fins. The absence of 

 traces of pectoral fins in Coccosteus can 

 not be used as a sure proof for the ab' 

 sence^of fins in the other Arthrodira. 

 We must remember that it is a usual 

 occurrence to find only fragments of the 

 pelvic girdle ; moreover the fin rays are 

 hardly ever found. 



If all the above mentioned circum' 

 stances make the presence of pectoral 

 fins in the Arthrodira presumable, there 

 are some others making it problematical. 

 For in some Arthrodires, as for instance 

 Acanthaspida from Spitsbergen (Heint?- 1929.1, .2) and Coccosteus angustus (Bryant 

 1929) from the Portage shale, N. Y., the body carapace extends very far downward and 

 no cleft between the dorsal and ventral armor is developed. If, in these forms we place 

 the pectoral fins behind the body carapace, they will be too far back and too close to 

 the pelvic fins. The newly found Acayithaspida from Germany (Broili 1929-1930) shows 

 an unusually perfect preservation; nearly the whole shell-covered body can be perfectly 

 seen. Also the dorsal fin (or spine) is well preserved. But no traces of pectoral fins can 

 be established. Moreover the pelvic fins are also absent. 



Thus, nothing positive can be said about either the presence or absence of pectoral 

 fins in the Arthrodira. We must wait for new and better preserved material to solve 

 this question. 



Finally, however, a few words may be said about the pectoral spines of Dinichthys 

 described by Newberry (1889, p. 144, PL V). These remarkable long bones, usually found 

 together with Dimchthys remains, belong, without doubt, to this fish. Newberry thought 

 that they corresponded to the long spine-like ''Ruderorgan" described by von Koenen 

 (1883-1895) in Coccosteus from Germany. We now know that von Koenen's "Ruder- 

 organ" is the spinal, which is well developed in Dinichthys, as in all other Arthrodirans. 



Text-figure 86. 



Crushed fragments of an Arthrodire (not a Dtmchthys), 

 showing trace of pectoral fin (?). 



fr, fin rays; x, pectoral fin (?); L, left and R, right plates of armor. 



