372 



THE OSTRACODERMS. 



matrix not visibly affected by acids. The part of the atrial chamber not 

 occupied by the remnants of the viscera is always filled with a coarse grained sand 

 that evidently worked its way in through the atrial opening after the animal died. 

 When treated with dilute acids, this sandy matrix becomes friable and can often 

 be easily worked away, exposing the contour of the gills and viscera. 



FIG. 252. Head of Bothriolepis. A, The bony cranial plates have been etched away, leaving a natural cast 

 of the inner, neural surface of the head; B, bony plates covering the neural surface of the head, seen from the 

 inside. The matrix has been chiseled away. Photographs; slightly restored. 



is. p.at. 



Km. K' 



"civ I 



FIG. 253. Transverse sections of the head of Bothriolepis. The dermal bones black; the coarser, more sandy 

 matrix, dotted; the softer matrix of a chalky consistency, shaded. Slightly diagrammatic. 



Viscera. Above, or to one side of the gills was a large oblong stomach, the 

 contour of which was conspicuous, owing to its carbonized contents. The cloaca 

 was guarded by a thin rounded, cloacal scale that is faintly tuberculate on its free 

 or ventral surface and with concentric lines on its dorsal surface. The cloaca 

 opened into the posterior part of the atrial chamber about half an inch in front of 

 the posterior margin of the ventral shield. (Fig 251, cl.}. 



