AFFINITIES OF ORTHIS AND STROFHODONTA. 493 



of constancy than do the forms from Iowa, called 0. ioivensis, or the forms 

 of 0. impressa from the Ithaca zone. 



In the European localities, also, considerable plasticity is seen, especially 

 where the race is abundant. This is interpreted as indicating that the 

 Tully species is a recent arrival in the local fauna. 



The second species, Strophodonta perplana var. Tulliensis (figures 1-4, plate 

 12), is a mutation of the race which began in Stro])homena altemata in the 

 Trenton stage. In the Hamilton rocks immediately below the Tully, the 

 form is Strophodonta perplana ; in the Ithaca zone above, it is Strophodonta 

 mucronata, Conrad. This is followed by Strophodonta perplana var. nervosa^ 

 Hall, of the higher Ithaca and Chemung zones. 



Without going into details, the prominent points in the geologic mutations 

 are that the race beginning in the Trenton, Strophomena altemata, runs through 

 a number of species differing in the proportions of form but retaining the 

 structural features and surface markings with considerable constancy. At 

 the base of the Devonian, two races diverge from the stem ; other features 

 remaining alike, the one is a thin, flat, and but slightly curving form, the 

 typical Strophodonta perplana, Conrad. This appears to be an American 

 type and is seen with variations all through our Devonian, but it is not 

 described in the European Devonian. The other, beginning flat, in the 

 course of its growth more or less suddenly bends toward the dorsal valve. 

 This is the Orthis inter strialis, Phillips, of the European Devonian, and 

 Strophomena incequistriata, Conrad, of the New York Hamilton. The inter- 

 strialis race is recognized in our Chemung Strophodonta cayuta and in the 

 upper and middle Devonian of Europe and the east in Strophomena dutertril 

 and S. aselli. 



In the European race, as we reach the Cuboides zone, the terminations of 

 the hinge develop into slender, mucronate points. In the American race 

 these mucronate points first appear in the Tully limestone forms (figure 1, 

 plate 12), and are characteristic of the race afterward till it ceases. 



The representatives of this type of Strophomena are common in Europe 

 throughout the Devonian, going under the specific names interstrialis, aselli, 

 and dutertril, and the conspicuous development of the mucronate points did 

 not appear till about the stage of the appearance of Rhynehonella cuboides. 

 The valuable testimony for correlation furnished by the Tully Strophomena 

 is that although plainly, in its main features, an American race of its genus 

 up to the Tully limestone stage, from there upwards it shows affinity with 

 the European representative as it appears in Europe in the Cuboides zone 



and upward. 



The third species, Rhynehonella venustula, Hall (figures 4, 8, 14, 28, 24, 

 27, 21), 31-34, plate 13), is by common consent closely allied to R. cuboides 

 of Europe, the chief distinction lying in the number of plications in the 



