STROPHOMENIDiE. 281 



each lobe, in some species, being channelled out along the centre, while in Orthis it is 

 generally formed of one piece. In Strophomena it is situated directly between the dental 

 sockets, or having between them and it a small prominent ridge or brachial process (?), 

 for this last is scarcely developed where it exists, and forms a marked contrast to what 

 we find in the same valve of OrtJiis. There are also four more or less distinctly defined 

 muscular (adductor) depressions, which are longitudinally parallel to each other, and 

 separated by ridges ; while in Orthis these four divisions are placed in pairs one above 

 the other. The differences in the interior arrangements of the ventral valve in the two 

 genera are not so well marked ; they form in both cases a more or less defined saucer- 

 or fan-shaped depression, notched in front, and margined by a prominent rim. The 

 external surface is either striated or more or less coarsely ribbed. 



The genus Strophomena is represented by many species in our Silurian rocks ; but 

 for the full description of some of them material is still wanting, and will, no 

 doubt, upon further search be forthcoming. In the mean time we have been compelled 

 to provisionally separate certain forms which, when better known, may be considered 

 as synonyms of some of the others. 



Some few species of Orthis, it is true, by their exceptional shape, seem to lessen the 

 value of the distinctive characters which so clearly separate the generality of species 

 composing the two genera, but the interior muscular and other arrangements in both 

 appear to be sufficiently distinct. 



At p. 231 of his valuable manual, Dr. S. P. Woodward observes that there are no 

 apparent brachial processes in the dorsal valve of Strophomena, and that it is possible 

 that the spiral arms may have been supported at some point near the centre of the shell, 

 as in Productus ; and this may, perhaps, have been the case ; but there appears also 

 evidence, in some species, of the existence of rudimentary brachial processes, to 

 which the oral arms may have been attached. This subject will, however, demand 

 further study and research prior to arriving at any definite conclusion. 



Strophomena rhomboidalis, Wilckens (sp.). PI. XXXIX, figs. I — 21 ; and 

 PI. XLIV, fig. 1. 



CoxcHiTA RHOMBOIDALIS, Tf^UcIienfi. Nacliriclit von seltenen Verst., p. 77, pi. viii, 



figs. 43 and 44, 1769. 

 " Die dritte Anomiten Art niit bteiter Schlosskatite," Hupscli. Naturg. des Nie- 



derdeutchlands, vol. i, p. 15, 

 pi. i, figs. 7 and 8, 1781. 

 Anomites rhomboidalis, Wahl. Nov. Acta Upsal., vol. viii, p. 65, 1821. 

 Producta depressa. Sow. Min. Conch., p. 86, pi. cccclix, 6g. 3, July, 1823. 

 — RUGOSA, Hisinger. Act. R. Ac. Sc. Holm., p. 333, 1826. 



36 



