30 



Strophomena planumbona erindalensis, var. nov. 



Plate VII, Figures 11, 12, and 16 



Leptaena planumbona, Hall. Pal. New York, 1, 1847, p. 112, pi. 31, fig. 4. 



Strophomena planumbona, Emmons. Amer. Geol., 1, pt. 2, 1885, p. 198, pi. 11, fig. 2; p. 186, 



figs. 54-56. 

 Strophomena planumbona, Foerste. Bull. Sci. Lab. Denison Univ., 17, 1914, p. 26, pi. 2, figs. 



4a, 4b. 



The original description of Strophomena planumbona follows: — 



Shell resupinate, robust, length and breadth as nine to eleven; cardinal line straight, sud- 

 denly deflected at the extremities, equal to or greater than the width of the shell; sides a little 

 contracted just below the cardinal extremities, leaving slightly salient angles; ventral (dorsal) 

 valve flat or slightly depressed near the beak, elevated and very convex in the middle, some- 

 what abruptly and concentrically deflected towards the margin; dorsal (ventral) valve flat on 

 the disc, slightly elevated towards the beak and deflected to correspond to the other valve; 

 surface marked by radiating striae, every third, fourth or fifth of which is alternated by a stronger 

 one; entire surface (in perfect specimen) marked by fine concentric elevated lines and a few 

 imbricating lines of growth. 



Foerste {op. cit.) referring to the same species, makes the following remarks 

 as to the proportions of the shell and the number of striae: — 



In the specimen represented by Fig. 4a the length is 20 mm.; the width, 25 mm.; and the 

 convexity about six millimetres. In the specimen represented by Fig. 4b the convexity equals 

 seven millimetres and in the most gerontic specimens it may attain even 11 mm. Surface striae 

 fine and thread-like, about 13 in a width of five millimetres, occasionally as few as 11, rarely 

 as many as 20. 



Numerous examples of a variety of this species occur in the zone of Stropho- 

 mena varsensis in the Erindale member on Mullet creek. They differ from the 

 type of the species in the more evenly-rounded antero-posterior outline, and 

 in the finer striation. The dorsal valve of the variety from the Credit river 

 is more rounded in the portion anterior to the beak and is not so incurved along 

 the anterior margin, while the convexity of the valve is less than in the type. 

 The radiating striae are much finer in 5. planumbona erindalensis: they average 

 30 in five millimetres at the anterior margin, but certain specimens were found 

 in which the number reaches 35 in an equal distance. The striae may be 

 sub-equal or there may be an alternation of coarse and fine ones. On some 

 valves, it has been observed that three or four fine striae may group them- 

 selves between two coarse ones. By the aid of a good lens extremely fine con- 

 centric lines can be seen to cross the radiating striae. 



The average width of the Credit River variety is 18 mm. and the length 

 12.3 mm. The smallest specimen measures 12 mm. in width by nine millimetres 

 in length. The type specimen (a dorsal valve) is 28 mm. wide and 17 mm. 

 long, with a convexity of 2.5 mm. 



Examples of the above form were sent to Dr. Foerste whose opinion was 

 that they should be given a new varietal name, based on the flatter form and 

 finer striation. He also said that in many of the forms from the Credit river 

 which were already in his possession, the callosity bordering the margin of 

 the pedicle valves, a small distance back of their anterior and lateral edges, 

 was rather narrower than in typical S. planumbona. No valves showing good 

 interiors are contained in our collections. 



According to Foerste, the forms most closely resembling S. planumbona 

 erindalensis are found in the Waynesville division of the Richmond in Ohio 

 and Indiana. 



Locality. — Mullet creek, Erindale. 



No. 12174, Royal Museum of Paleontology. 



