THE ANTIARCHA. 



371 



ductive organs. The anterior part of the trunk 

 and the branchial portion of the head was sus- 

 pended from the roof of the branchiocephalic 

 shield by stout fibrous tissue attached to two 

 deep median processes, or ridges of cancellous 

 bone, that projected downward and forward 

 (Fig. 251, pr.) The sides and ventral surface, 

 were unattached and were surrounded by a 

 spacious peribranchial, or atrial chamber, which 

 opened outward between the sides of the body 

 and the ventrolateral walls of the branchiocep- 

 halic buckler. 



Atrial Frill. — The lips of the atrial opening 

 were guarded by a membranous frill (Figs. 247, 

 248) . Its ventral portion was broad, often folded 

 or pleated, and sometimes marked by faint, longi- 

 tudinal lines. It appears to project from the 

 inside of the chamber and to be attached to its 

 ventral wall by a faint transverse ridge. The 

 dorsal portion was much narrower, and appeared 

 to be a membranous extension of the posterior 

 and lateral margin of the shield. The contents 

 of the branchiocephalon are never squeezed out 

 of the posterior end of the buckler. Sections 

 of many different specimens always show the gills 

 and stomach in their proper position, even when 

 the shields are crushed almost flat. If the animal 

 lies ventral side down, the viscera always lie 

 on the ventral wall. If it died ventral side up, 

 they lie on the dorsal shield, and in all cases well 

 in front of the atrial opening. 



Gills. — Seven pairs of broad lamellate gills 

 were present on the sides of the head, in about 

 the center of the branchiocephalon. They 

 appear as seven pairs of thin, parallel, black 

 lines, somewhat wavy, with minute black spots. 

 (Fig. 256.) A more conspicuous dark band, or 

 a clear space, separates the gills of one side from 

 those of the other. Altogether they form an ob- 

 long nodule about an inch and a half to two 

 inches long, one inch broad, and from an eighth 

 to a quarter of an inch thick. The nodule 

 always consists of a peculiar fine-grained, soft 



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