380 ROBERT TRACY JACKSON ON THE 



the ligament joins the valves on the hinge line and a triangular cartilage pit extends 

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

 ]"). 329. Teeth may exist on the hinge line as in jVvicuUi and Perna, oi* not, as in Pecten. 

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

 tial stages of the dissoconeh growth on the liorder of the prodissoconch valve, thence 

 progressively produced to the free border of the valves, fig. 33, p. 330; PI. xxvii, figs. 

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



The form of the 3'oung shell as above described is found to be closely similar to that 

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

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

 eai'ly forms ditfer 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 Avhich are here referred to. I would, therefore, 

 introduce a new genus — Rhombopteria. 



Rhombopteria, gen. nov.' A group of Lower and Upper Silurian Aviculoids, in which 

 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 postei'iorly 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 '\\\ RhombojAeria (Av.) glabra, sp. Miuist. 

 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- 

 vsible, 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 — JRlwmbopteria (Av.) intra, 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 ]N[. Barrande come under the genus Rhombopteria, among which 

 should be mentioned: R. vseudomira, sp. Barr.; R. cognata, sp. Barr.; 

 R. scala, sp. Barr.; R. patricia, sp. Barr. and R. glabra, sp. Munst, all 

 of Avhich are figured on Plates 226-228 inclusive. 



Rhombopteria is the nearest approach to a primitive Aviculoid that 

 has been found in studying fossil species. Not only can the nepionic 

 E (upper Silurian), stagcs of modcru Avicula, Pecten and Perna be closely compared to 

 systeme Siiurien. ^j j type, but camera drawings of the young of Aviculopecten, Lepto- 



( After Barrande.) ^ \ . , . , .,,"-, , , 



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

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

 trentonensis, Con., A.? gregaria, Ulrich, and A.? demissa. Con., are near allies of Rhom- 

 bopteria, 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 Rhombopteria. In the comparisons 

 of young Avicula, Pei-na, Pecten, etc., to Rhombopteria, it will be observed that there 



/"'/V'"') a rhonil), and Tm/xi:'^ a Aving. 



Fig. 51 — Bhombop- 

 teria mira, formation 



