68 



Ilionia Galtensis, Whiteaves. 



Plate 15, fig. 3. 



Ilionia Oaltensis, Whiteaves. . . .1884. This volume, pt. 1, p. 15, pi. 3, figs. 1, 1, a-b. 

 " S. A. Miller.. ,1889. N. Am. Geol. and Palajont., p. 483, fig. 833. 



This species was based upon two very imperfect specimens, the one 

 collected at Gait by Dr. R. Bell, in 1861, and the other at Durham by 

 Mr. Townsend between the years 1878 and 1882. Neither of these speci- 

 mens gives a correct idea of the marginal outline of the valves or of the 

 nature of the surface ornamentation. Since the first part of this volume 

 was written, Mr. Townsend has obtained five additional and more perfect 

 examples of /. Galtensis at Durham, and, quite recently (in 1894), the 

 unusually perfect and well preserved left valve from Elora figured on Plate 

 15. This specimen, which is drawn from a gutta-percha impression of a 

 natural mould of the exterior of that valve, shows that, when perfect, the 

 valves are nearly as high as long and that the ventral margin is narrowly 

 rounded and somewhat produced at or a little in advance of the mid- 

 length. The beaks are small and subcentral, and the surface markings 

 are seen to consist of very numerous and closely disposed fine concentric 

 strise, with a few coarser ones at irregular but more distant intervals. 



The systematic position of Ilionia is quite problematical, as the muscular 

 impressions are not at all well preserved in any of the specimens collected, 

 and its hinge dentition is still unknown. Zittel, in the first part of the 

 second volume of his Handbuch der Pateontologie, published in 1881, 

 places it between the Pholadomyidse and the Anatiaidse, in a group 

 characterized as " palaeozoic genera with a simple pallial line, and of doubt- 

 ful systematic position." Tryon, in the third volume of his " Structural 

 and Systematic Conchology,"published in 1884, places it in the Anatinidse, 

 and Fischer, in the last " fascicule " of his Manuel de Conchy liologie, 

 published in 1887, includes it in the Lyonsiidse. On the other hand, the 

 external form of the Elora specimen of /. Galtensis figured on Plate 15, is 

 so similar to that of some of the lower Palaeozoic species of Lucina (inclu- 

 sive of Paracyclas) that it is difiicult to see how Ilionia is to be distin- 

 guished from Lucina, and in this connection it is to be observed that the 

 Tellina prisca of Hisinger, which E. Billings states is a typical Ilionia, 

 is regarded by Bronn and Ferdinand Rcemer* as a Lucina. 



Ilionia (?) costdlata, Whiteaves. 

 Ilionia (? ) costvlata, Whiteaves. 1884. This volume, pt. 1, p. 15, pi. 2, fig. 5. 



Elora, T. 0. Weston, 1867 ; two specimens : and Durham, J. Townsend, 

 1879-1883; one specimen. No additional examples of this species have 

 been obtained, since it was first described in 1884. 



*In the explanation of Plate 14, figs. 2, a-6, of the first vol. of the Lethaea Geognostica. 



