600 Systematic Paleontology — rppER Devonian 



Length 15-20 mm.; width 35-60 mm. 



The presence of fine interrupted radiating strife has not been noted 

 in the pi;blished descriptions of this form but H. S. WiUiams informs 

 the writer that they are observed in well-preserved species of S. miicrona- 

 itis, S. mucronaius var. posterns^ and S. (Dclthyris) mesacostalis in New 

 York. 



This variety is subject to considerable variation in size and form. The 

 specimens observed in the lower beds containing the Ithaca fauna in 

 Maryland are small and, so far as seen, without a median plication in 

 the sinus, agreeing closely with those found in the Ithaca fauna of central 

 Xew York. In the Liorliynclius glohuUfonnc zone the specimens attain 

 a much larger size than is usual in individuals in central New York, while 

 some possess a plication in the bottom of the sinus, a feature not seen in 

 the New York forms. The latter feature is, however, not constant. Some 

 specimens have the lateral margins nearly at right angles to the hinge- 

 line being then identical with a form described as S. dactylus by H. S. 

 AVilliams from the Liorhynchus glohuliforme zone of eastern New York. 

 These modifications are so closely connected by transitional forms in 

 Maryland that it has not seemed possible to discriminate them by varietal 

 names. All are clearly mutations of S. mucronaius of the Hamilton. 



This variety may closely approach iS. mua-onatus in its general pro- 

 portions. Usually, however, the specimens are somewhat smaller and pos- 

 sess more extended cardinal angles upon which the plications are obsolete. 



The variety also closely approaches 8. {Deltliyris) mesacostalis in its 

 external features, but difilers in the absence of a median septum iu the 

 interior of the ventral valve. A few individuals have been observed pos- 

 sessing a very rudimentary septum suggesting a transition to the latter 

 species. 



This is one of the most abundant and characteristic species of the 

 Ithaca fauna of Maryland and adjacent areas. In several localities it 

 may recur in the Tropidoleptus fauna of higher horizons though it is 

 very rare and the stratigraphic relations are obscure or identification is 

 insecure in these rases. When assuredly identified it is confined to the 

 Ithaca fauna. 



