Report of TEE State GEOLOGIST. 569 



communicating with the interior of the .shell by a tubular canal, 



which narrows slightly as it passes inward. No cardinal area pf 

 deltidium is present, the growth-lines passing between the beak 

 and the posterior margin as elsewhere on the shell. Brachial 

 valve depressed-convex; beak marginal; posterior margin regu 

 larly arched and thickened. Interior of pedicle-valve with 

 muscular impressions confined to the umbonal region. Adjacent 

 to the opening of the sipho, and just within the cardinal margin 

 lie two elongate scars which are accompanied on either side by 

 broader, somewhat expanded or flabellate, simple and less dis- 

 tinct impressions. Directly in front of the middle pair and in 

 the axis of the shell lies a small central scar, and at either side of 

 it a transversely elongate impression. These latter impressions 

 are distinctly separated from the former by a transverse ridge. 



In the brachial valve the impressions are equally concentrated, 

 the entire muscular area being bounded on its posterior margin 

 by a prominent ridge which, at the sides, merges into a compound 

 lateral scar. The central portion of the area is much depressed 

 and is divided axially by a narrow ridge or septum. 



Shell ornamented with concentric lines and ridges, the epider- 

 mal layer bearing hollow spines, which are distended at the base. 

 Shell-substance calcareo-corneous. the layers beneath the epider- 

 mis being punctured by radiating and branching tubules. These 

 layers are concentric and not parallel to the internal surface. 



Type, Siphonotreta ungiticidata, Eichwald. 



Distribution. Lower — Upper Silurian (Baltic Provinces, 

 England. Scotland, New York, Minnesota). 



Schizambon, Walcott. 1884. 



(Plate 4, Figs. 16-20.) 



Shell ovate or oblong ; pedicle- valve the more convex, with a 

 short obtuse marginal beak. Foramen in front of the beak, oval in 

 outline and connected with the apex by a tapering groove or 

 progressive cicatrix. Interior of both valves with faint diverg- 

 ing lateral ridges indicative of muscular areas. 



Surface covered with concentric growth lines which may be 

 spiniferous. Shell substance calcareo-corneous. 



Type, Schizambon typicaMs, Walcott. (Lowest Silurian 

 [Primordial ?]). 



1891. 7-2 121 



