The Class Ostracophori 



579 



Ostracophores, two families, Thelodontidce and Birkeniidce, being 

 there represented. 



Order Anaspida. Recently a fourth order, Anaspida (a, 

 without; aa-Ttis, shield), has been added to the Ostracophori 

 through the researches of Dr. Traquair. This group occurs 



FIG. 363. Pterichthyodes testudinarius Agassiz, side view. (After Zittel, etc.) 



in the Upper Silurian in the south of Scotland. It includes 

 the single family Birkeniidce, characterized by the fusiform 

 body, bluntly rounded head, bilobate, heterocercal tail, and a 

 median row of hooked spinous scales along the ventral margin. 

 No trace of jaws, teeth, limbs, or internal skeleton has been 



FIG. 364. Birkenia elegans Traquair. Upper Silurian. (After Traquair.) 



found. Unlike other Ostracophores, Birkenia has no cranial 

 buckler with orbits on the top, nor have the scales and tubercles 

 the microscopic structure found in other Ostracophores. In 

 the genus Birkenia the head and body are completely covered 

 by tubercular scutes. The gill-openings seem to be represented 

 by a series of small perforations on the sides. A dorsal fin is 

 present. Birkenia elegans is from the Ludlow and Downstonian 

 rocks of southern Scotland. Lasianius problematicus from the 

 same rocks is very similar, but is scaleless. It has a row of 

 ventral plates like those of Birkenia, the only other hard parts it 



