No. 444.] P TKRASPI D.K AND CEP HA LA SPIDjE. 849 



muscle ridge on their inner surface and the thickened, inturned 

 median ends, that were probably armed with chitinous, or horny 

 sheaths, show that they acted against one another in the median 

 line, either as cutting, or crushing jaws. Their structure and 

 position show in the clearest manner that the mouth must have 

 been situated between these mandible-like plates, not in front of 

 them. The presence of these remarkable structures around the 

 mouth of the ostracoderms shows more clearly perhaps than 

 anything else how wide the gap is between them and the true 



Appendages.— Powrie was the first to call attention to the 



laspis. They were later described 'md figured In I .ankester'and 



their structure which has caused them to be preserved." They 

 are characterized by a kind of reticulate or areolate marking and 

 although they show no trace of fin rays, they were regarded as 

 a remarkable kind of pectoral fin, " efficient in causing currents 

 Of water to pass to the branchial organs." 



specimens of C. vunehisonii and makes the following statement 

 in regard to them: (Cat. B. Museum, p. 186). "A newel 

 point of much general interest is elucidated by the middle layer 

 of the shield, which is well preserved in several specimens." 

 " The present specimens prove distinctly that it extends back- 

 wards as a pair of postero-lateral < flaps ' beyond the rest of the 

 shield." ''The outer layer is broken away, so that direct con- 

 tinuity can be observed between the appendage and the middle 

 layer." "The structures are merely a portion of the shield 

 itself, divested of the outer and inner layers to insure flexi- 

 bility " (p. 187). 



After quoting Lankester's opinion of them, he adds, "Some 

 connection with the 



ipendages in the 



