284 APPENDIX TO THE SUPPLEMENTS TO 



corhynchus, and Dr. Barrois observes that their generic characters agree well with those 

 observable in the A. Bavidsoni ; his A. Ussensis being the species that most nearly 

 resembles the shell under description. 



Dr. Barrois says, in his valuable work above referred to, that we must also 

 compare A. Bavidsoni with the Productus elegans, M'Coy, as well as with the Chonetes ? 

 millepunctata of Meek and Worthen, of which specimens of medium size ornamented with 

 sixty ribs are identical with the Spanish form. He adds that Messrs. Meek and Worthen 

 hesitated, as did their predecessors, to place this shell in the genus Chonetes^ and, 

 remarking that it differs from true forms of Chonetes by its coarsely punctate structure, 

 proposed for it the new generic name of Isogramma, being at the time ignorant of 

 Dittmar's prior name — Aulacorhynchus. The punctured structure of the American 

 specimens is so coarse that the interior of the valve, seen with the lens, recalls the 

 disposition of the cells of small Chaetetes, the perforations, says Barrois, being so unusually 

 close that the intervals separating them are smaller than their diameters. 



These large punctures, adds Dr. Barrois, are equally observable on the specimens 

 from the Asturies ; that this structure is especially comparable to that of the Rudistes, it 

 being more prismatic cellular than punctate ; the prisms are perpendicular to the lamellae 

 of the shell, and often delicately subdivided. The medium diameter is of one sixth of a 

 millimetre. The shell is rather thick, attaining in the larger number of specimens one 

 millimetre. The thick cellular layer of the shell is covered outwardly by a thin, smooth 

 layer, which shows no traces of perforation. So that in reality the perforations 

 observable on the inner surface of the valves do not traverse the entire thickness of the 

 valve, and in this respect agree with what has been found to be the case in Chonetes, 

 Productus, and some of the Strophomenidee. 



The interior muscular and other markings are described by Dittmar, P. v. Semenow, 1 

 and C. Barrois ; and they differ notably from what we find in Chonetes. I have in PL 

 XX, fig. 22, reproduced a figure of a ventral valve, after Barrois, showing the muscular 

 impressions. 



1 " Ueber die Fossilien des Sclilesischen Kohleiikalk.es ;" 'Deutsch. Geolog. Gesellschaft,' 1854. 



