78 Trans. Acad. Set. of St. Louis. 



area high, flat or slightly convex or concave, sharply defined 

 along the margin, sloping forward from the hinge-line at an 

 angle of about 30° with the plane of the valve ; the mesial 

 sinus broad and deep, not plicated, rounded in the bottom, 

 not sharply defined, produced anteriorly in a lingual exten- 

 sion ; the lateral slopes very narrow ; in the sandstone casts 

 the syrinx is usually well exhibited. Brachial valve somewhat 

 gibbous in the middle, the lateral slopes concave from the 

 deepest point of the valve to the cardinal extremities, flat- 

 tened towards the cardinal angles; the mesial fold prominent, 

 rounded above, sharply defined, not plicated, deeply emar- 

 ginate in front. Surface of each valve marked with fifteen 

 to twenty simple rounded plications on each lateral slope, grow- 

 ing smaller towards the cardinal extremities ; in addition to the 

 plications each valve is usually marked by several more or less 

 conspicuous lines of growth which in the pedicle valve are 

 continuous across the cardinal area. The dimensions of an 

 average specimen are: length, from beak of brachial valve to 

 the anterior margin of the fold and sinus, 14 mm., length of 

 hinge-line, 41 mm., height of cardinal area, 15 mm. An- 

 other large brachial valve has a length of 17 mm., and a 

 breadth along the hinge-line of 63 mm. 



BemarJcs. It has always been found difficult to draw 

 sharply defined specific lines between the species of the genus 

 JSyringoihyris. From the Kinderhook beds at Burlington two 

 species of the genus have been described, /S. exienuatus Hall, 

 and S. halli Winchell. HalFs species is from the Chonopectus 

 sandstone ; it was originally described from the brachial valve 

 alone, but in recent collections from Burlington several specimens 

 preserving both valves have been secured and it is a common 

 species in the fauna. The types of S. halli are not from the 

 Chonopectus sandstone but from the overlying limestone, 

 although specimens of 8. extenuatus from the sandstone are 

 present in the University of Michigan collection labeled 8. 

 halli var., by Winchell. From a careful study of the types 

 of 8. halli and of a good collection of 8. exienuatus, it is 

 believed that the two species are not identical as has been 

 sometimes suggested. As they occur at Burlington 8. halli 

 is always smaller than 8. extenuatus, the largest specimen of 



