380 ROBERT TRACY JACKSON OX THE 



the lig'ainent joins the valves on the hhige line and a triangular cartilage pit extends 

 inwards from the limits of the prodissoconch valve as shown in young Perua, fig. 30, 

 p. 329. Teeth may exist on the hinge line as in Avicula and Perua, or not, as in Peeten. 

 The i-ight valve in the young of these genera has a byssal sinus originating in the ini- 

 tial stages of the dissoconch growth on the border of the pi-odissoconch valve, thence 

 progi'essively produced to the fi'ee border of the valves, flg. 33, j). 330; PI. xxvii, figs. 

 2, 3, 7, 8, 9, and PI. xxviir, fig. 10. 



The form of the young shell as above described is found to be closely similar to that 

 characteristic of the adults of many Avlculoids from the Lower and Upper Silurian for- 

 mations. Especially is it comparable to early Aviculoids figured by Barrande. These 

 early forms dilfer widely from Avicula and require one or more distinct generic names. 

 The Devonian Aviculoids have been subdivided generically by Professor Hall, but none 

 of his genera suffice to cover the types which are here referred to. I would, therefore, 

 inti'oduce a new genns — Rhombopteria. 



Khombopteria, gen. nov.^ A group of Lower and Upper Silurian Aviculoids, in whicli 

 the shell is sub-rhomboidal, oblique, with umbos in the anterior half. The hinge line is 

 straight, less than the length of shell, and is commonly produced anteriorly as well as 

 posteriorly from the umbos. The borders of the valves posteriorly descend in a curved 

 line from the terminus of the hinge without having any posterior extension of the wing. 

 Anteriorly the borders of the valves descend from the hinge, forming an auricle close to 

 the hinge line. In some species the auricle may be in close proximity to the umbos with- 

 out any anterior extension of the hinge line, as m Rhomhopteria (Av.) glahra, s|). Munst. 

 Two adductor muscles and lateral teeth exist in closely related forms and probably are 

 characteristic of this genus; but the features are not definitely known. It is quite pos- 

 sible, also, that a cartilage pit exists, although it also is not known, as the internal char- 

 acters of the genus have not been ascertained. Type — lihombopteria (Av.J mira, sp. 

 Barr., fig. 51. For other figures of this species see Barrande's Systeme Silurien de la 

 Boheme, Vol. VI, PI. 220. Besides the type several species figured 

 by M. Barrande come under the genus Khombopteria, among which 

 should be mentioned: M. pseudomira, sp. Barr.; li. cognata, sp. Barr.; 

 JR. scala, sp. Barr.; H. patricia, sp. Barr. and J?, glabra, sp. Munst, all 

 of which are figured on Plates 226-228 inclusive. 

 '^i'iifc.^ Rhombopteria is the nearest approach to a primitive Aviculoid that 



Fig. oi —Bhomhop- j^,^^ been fouud in studying fossil species. Not onlv can the neijionic 



teria mira, formation . " 



E (upper Silurian), Stages of modern Avicula, Peeten and Perna be closely compared to 

 systeme Siiuneu. .j.j.jjg type, but camera drawings of the young of Aviculopecten, Lej^to- 

 desma and other paheozoic and mesozoic allied genera bear a close re- 

 semblance to Rhombopteria as well. The American Lower Silurian species, Avicula? 

 treutonen.-ii,s, Con., A.? gregaria, Ulrich, and A.? deniissa, Con., are near allies of Rhom- 

 bo])teria, especially the species trentonensis. In Aviculoids as a rule, the younger the 

 specimen or the older the geological formation from which a specimen is obtained the 

 nearer does it approach to the features of the genus Rhomboptei'ia. In the comparisons 

 of young Avicula, Perna, Peeten, etc., to Rhombopteria, it will ])e observed that there 



1 '/j(/!ii3<i<;, ;i rhoml), and -ts/iov^ a wing. 



