PATERINID^. 367 



Genus HELMERSENIA Pander. 



Siphonotreta Jeeemejew, 1856, Verhandl. Russ.-kais. min. Gesell. St. Petersburg for 1855-6, No. 2, pp. 73 and 80. 



(New species S. ladogensis described in German.) 

 Helmersenia Pander, 1861, Bull. Acad. imp. sci. St.-Petersbourg, vol. 3, columns 48-49. (Jeremejew's species 



described and discussed in German.) 

 Helmersenia Pander, Zittel, 1880, Handbuch der Palasontologie, Bd. I, Abth. 1, p. 666. (Described in German.) 

 Keyserlingia Karpinsky, 1887, Bull. Acad. imp. sci. St.-P6tersbourg, vol. 31, p. 476, footnote. (The use of Keyser- 



lingia in this footnote is clearly a typographical error, Helmersenia being intended. The genus is discussed 



in German.) 

 Helmersenia Pander, Oehlert, 1887, Manuel de conchyliologie, by Fischer, p. 1264. (Described in French, see p. 369.) 

 Helmersenia Pander, Hall and Clarke, 1892, Eleventh Ann. Kept. State Geologist New York for 1891, p. 254. 



(Described.) 

 Helmersenia Pander, Hall and Clarke, 1892, Forty-fifth Ann. Rept. New York State Mus. for 1891, p. 570. (Copy 



of preceding reference.) 

 Helmersenia Pander, Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, p. 119. (Copies Hall 



and Clarke, 1892a, p. 254, and discusses genus.) 

 Helmersenia Pander, Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 4, PI. XI, and pp. 142 and 143. (Classi- 

 •> fication of genus.) 



General form longitudinally ovate, subcirculax to transversely ovate, with the ventral 

 valve broadly subacuminate and dorsal valve broadly rounded except at the minute beak; 

 ventral valve moderately convex, rising on the umbo and sloping most rapidly down to the 

 minute, nearly marginal beak; dorsal valve gently convex with apex maz'ginal. 



The interior of the ventral valve shows a narrow area with a thickened median ridge ex- 

 tending a short distance forward to the visceral area. The latter has the general form of the 

 visceral area of Oholus, with the muscle scars in front on each side of the center. One specimen 

 shows the central and the outside and middle lateral scars; beneath the area, at the base of the 

 median ridge, the main vascular sinuses arch outward and then forward; between the vascular 

 sinus and the outer margin of the shell there are indications of the transmedian and anterior 

 lateral muscle scars. The interior of the dorsal valve shows a slender median ridge extending 

 well forward into the valve, and the main vascular sinuses well out toward the side of the shell. 

 None of the specimens show the muscle scars of this valve, and I have not been able to find 

 more than the broken margin of the area. 



Surface marked by concentric strire and a few stronger lines of growth. Scattered irreg- 

 ularly over the surface on the epidermal layer there are elongate bases of numerous slender 

 spines that closely resemble the bases of the spines of Siphonotreta verrucosa (Eichwald). The 

 inner surface is minutely punctate with a few larger scattered punctse. 



The shell varies in size from 2 to 3 mm. in diameter. It rarely exceeds 2 mm. 



Type. — Siphonotreta ladogensis Jeremejew. 



From Pander's remarks [1861, p. 48] in describing the genus Helmersenia it is evident that 

 he considered the shells described by Jeremejew [1856, p. 73] as Siphonotreta ladogensis as belong- 

 ing to this genus. As pointed out by Karpinsky [1887, p. 476] the form studied by Pander 

 differs from S. ladogensis in being more circular in outline. I strongly suspect, however, that 

 if the types of S. ladogensis were compared with the material used by Pander they would be 

 found to be specifically identical. The figures given by Jeremejew [1856, p. 73] in my opinion 

 represent young shells of the form subsequently named by Karpinsky. 



Through the courtesy of Dr. Fr. Schmidt, of the Geological Survey of Russia, I have had 

 the opportunity of studying specimens of Helmersenia collected by Karpinsky at Kunitz, in the 

 Government of Pskow. I also received from Dr. A. Mickwitz some fine specimens collected at 

 Koporje, in the Government of St. Petersburg. With the specimens from Kunitz there some- 

 what rarely occurs Keyserlingia iuchi (de Verneuil) and large numbers of Oholus' (Schmidtia) 

 celatus Volborth. The associated forms at Koporje are Oholus (S.) celatus and fragments of a 

 larger Oholus. 



Pander [1861, p. 48] describes the ventral valve of tliis species as having at the top of the 

 beak a circular opening that does not, as in the " Siphonotretem," extend into a canal. I have 

 carefully studied with a strong lens more than one hundred specimens of the ventral valve of ' 



