Troimdoi.ei'I'Us Fauna at Canandaigua Lakk, N. \'. lol 



referred to these two varieties, while the other species, O. belliilus 

 Clarke, is represented by about 60 specimens. A number of the 

 smaller individuals retain both valves, but most are separated and 

 there are about as many of one valve as the other. 



Hall, in 1867, referred all the specimens of this genus found in the 

 Upper Helderberg, Hamilton, and Chemung, of New York, to the 

 single species Streptorhynchiis (■heiuiingensis and divided it into four 

 varieties, S. pandora from the Schoharie and Corniferous, X arcto- 

 striata from the Hamilton, S. pervctsa, Hamilton, and S. pecteuacea for 

 the Chemung. He stated, however, that there exist such gradations be- 

 tween these that it is extremely difficult to distinguish them and that 

 specimens exactly like S. pandora are found in the Chemung, while 

 the varieties arctostriata and perversa are not confined to the Hamil- 

 ton. Schuchert, in 1S97, divided the group by making two species 

 and two varieties. Orthotlietes ehemungensis, including Hall's variety 

 pectcnacea, is restricted to the Chemung specimens. O. e/ie/nitngensis 

 arctistriatus to the Hamilton, O. e/ieinungensis per'oersiis to the 

 Hamilton and Corniferous, and O. pandora Billings to the Upper 

 Helderberg. 



The differences between the varieties seem to have been based largely 

 on surface ornamentation. From the descriptions given in Vol. IV, 

 Pal. N. Y., the following principal characters are taken. 



" Such forms as S. arctostriata have the strire nearly equal and quite 

 thin and prominent, but even then there are fine intermediate striae, 

 but the interspaces are so nearly ecjual that the surface presents a pretty 

 uniform aspect, though there are many gradations. Specimens of this 

 kind have usually very broad convex dorsal valves which are some- 

 times depressed in the center." 



5. pecteJiacea. — " Surface marked by distant prominent striae with 

 two or three finer ones between. Well marked specimens are usually 

 small or medium size." 



Numerous specimens of these varieties are figured. The majority 

 of them are much larger than any in the present collection. Hall states 

 that the usual size of S. arctostriata is from one-(]uarter to three- 

 quarter inches in length and the width one-quarter to one-third greater. 

 The smallest specimen figured is 7 x 13 mm. and the average is from 

 16 X 25 mm. to 25 X 40 mm. It will thus be seen that the material 

 here described consists mainly of immature and dwarfed specimens. 



