326 



SCIENCE. 



[Vol. II., No. 31. 



While the interior structure of the hinge, and the 

 muscular and vascular marlcings, were now pretty well 

 known for most of the generic forms in use, com- 

 paratively little attention had been given to the 

 minute structure of the shell. Little more had heen 

 done than to show that some forms possessed a punc- 

 tate and others a fibrous texture. 



The study of this structure had been commenced 

 by him many years ago, but he had been thwarted in 

 his efforts to procure the required cutting and polish- 

 ing of specimens of the shells for microscopic study. 

 He had now been able to obtain such thin slices of 

 the shell as were required for this purpose, and had 

 already several hundred slides prepared for the mi- 

 croscope. 



A few of these only were shown, exhibiting the 

 shell structure of as many geuera. A considerable 

 number of photographs had been made, illustrating, 

 in a very satisfactory manner, the minute structure 

 of each one, enlarged to twenty diameters. The 

 photographs exhibited were illustrations of several 

 species of Orthis, Leptaena, Strophomena, Stropho- 

 donta, Chonetes, etc. 



The study of this shell-structure has shown very 

 satisfactorily, what was partially known before, that 

 the genus Orthis, as now defined and constituted, 

 includes very heterogenous material. External form, 

 hinge characters, and interior muscular impressions, 

 have been the chief guide; and yet forms have been 

 included under this genus, showing widely different 

 muscular markings. On further microscopic study, 

 it has been found that these differences in form of 

 muscular imprints are accompanied by important 

 differences in the shell structure. 



These differences may be noted in the illustrations 

 presented, where the shell of Orthis biforata, O. bo- 

 realis, O. tricenaria, O. occidentalis, O. flabella, are 

 non-punctate and coarsely fibrous. Orthis (?) stro- 

 phomenoides is, like Streptorhynchus, fibrous. Orthis 

 subquadrata has, like 0. occidentalis, a few large 

 punctae. 



In the second group, Orthis testudinaria, O. Vanu- 

 xemi, O. perveta, O. penelope, O. elegantula, O. clytie, 

 and O. hybrida, have one or more rows of punctae to 

 each ray, the rows well defined, and the intermediate 

 shell finely fibrous. 



The third group, consisting of Orthis multicostata 

 of the lower Helderberg, O. jowensis of the Hamilton 

 group, O. tulliensis of the Tully limestone, 0. im- 

 pressa of the Chemung group, are highly punctate 

 with a fine fibrous texture of the shell substance. 



The punctae usually come out along the summits 

 of the radiating striae or plications of the shell. In 

 some species the minute tubes perforating the shell, 

 and producing these punctae, bifurcate and diverge 

 before coming to the external surface of the shell. 



Tbis difference in shell structure, in forms known 

 as Orthis, will require a separation of the species into 

 groups based upon the shell structure, and character 

 of muscular impressions. Already we see that the 

 shells of compact fibrous texture have a form of mus- 

 cular impression quite unlike those with the punc- 

 tate structui-e; and we shall probably find that all 



the interior modifications of the muscular system are 

 accompanied by differences in the microscopic struc- 

 ture. 



This method of deterrainins the shell structure, in 

 cases where the specimens may be imperfect, and 

 thereby enabling the determination of obscure or 

 fragmentary material, and its geological relations, 

 will be of much importance to the geologist. 



The structure of the shell in Strophomena is closely 

 fibrous, with distant large punctae. In Stropho- 

 donta, the punctae are more numerous. In Chonetes, 

 the punctae are large, and arranged parallel to the 

 radii, having a pustulose aspect. 



In many other forms, the punctate texture of the 

 shell is characteristic, and of importance in the de- 

 termination of the generic forms. 



The physiological significance of this peculiar shell 

 structure will be considered upon some future occa- 

 sion, illustrated by more numerous examples. 



Rhizocarps in the paleozoic period. 



BY J. W. DAWSON OF MONTEEAL, CAN. 



The author referred to a previous memoir, entitled 

 'Spore-cases in coal,' published in 1871. This de- 

 scribed fossil remains in a shale from the Erian for- 

 mation at Kettle Point, Lake Huron, supposed to be 

 on the horizon of the Marcellus shale of New York. 

 The remains are minute brownish discs scarcely more 

 than one-hundredth of an inch in diameter. They 

 were recognized as probably spore-cases or macro- 

 spores of some acrogenous plant. The shale also con- 

 tains vast numbers of granules, which may be escaped 

 spores or microspores. In 1SS2 Dr. Dawson's atten- 

 tion was called to the discovery of similar bodies in 

 vast numbers in the Erian and lower carboniferous 

 shales of Ohio. The discoverer. Professor Orton, re- 

 garded these bodies as spore-cases, and as the chief 

 source of the bituminous matter in those shales. 

 Professor Williams found similar bodies in the Ham- 

 ilton shales of New York; and Prof. J. M. Clarke, 

 in the Genesee shale and in the corniferous lime- 

 stone. The last named are of larger size than the 

 others. 



No certain clew had been thus far afforded to the 

 affinities of these widely distributed bodies. But last 

 March, specimens were found in the Erian formation 

 of Brazil, by Mr. Orville Derby, which threw new 

 light on the subject, containing as they did, along 

 with the Spiirangites, abundant fronds of Spirophy- 

 ton. The Sporangites of Brazil resemble in every 

 respect the involucres or spore-sacs of modern rhizo- 

 carps, and especially the sporocarps of the genus Sal- 

 vinia. 



Dr. Dawson describes with technical exactness two 

 leading types which he has named provisionally Spo- 

 rangites brazillensis and S. bllobatus. The paper 

 offers the suggestion that these plants, now so insig- 

 nificant, culminated in the paleozoic age, and, occu- 

 pying the submerged flats of that period with abun- 

 dant vegetation, produced a great quantity of the 

 bituminous matter found in resulting beds. A rich 

 rhizocarpean vegetation in the early paleozoic and 



