94 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



along the anterior and sometimes also the posterior margin. In frac- 

 tured specimens the central longitudinal cavity is seen to be infiltrated 

 with a white silicious substance. 



In Plate 6, Figure 3, is shown a very interesting pectoral fin preserved 

 in counterpart (Yale Museum Cat. No. 295), and retaining the actino- 

 trichia in natural association with the spine. The latter is preserved 

 for a length of 8 cm., has a width of 8 mm., and thickness of 4 mm. 

 The fibrous rays are quite long and numerous as compared with other 

 species, and extend well up towards the proximal end of the spine. 

 There is no trace here of a basal cartilage abutting against the side 

 of the spine, owing to deficient preservation in this region, hence the 

 present specimen is unable to throw any light on the partly conjectural 

 restorations of Reis,-^ Fritsch,^ Jaekel,^ and others. In view of the ex- 

 treme interest attaching to the endoskeletal structure of the paired fins, 

 it is tantalizing to find -just those parts missing which are most needed 

 to clear up certain problematical details. Neither does this specimen 

 display any of the dermal granules with which the fin-membrane was 

 stiff'ened, but these are well exhibited in a smaller fin, possibly 

 identifiable as the dorsal fin of a young individual, shown in Plate 5, 

 Figure 3. In this specimen the limit of the exoskeletal part, or which 

 is the same thing, the outHne of the body-wall, is very distinctly shown. 

 Although the spine is only 2.5 cm. long, it is about as wide in propor- 

 tion as the adult spines. The smaller fin is preserved in counterpart 

 like the majority of fossil remains found in concretions. 



The specific title is dedicated to the honored and enduring memory 

 of the late Professor Othniel Charles Marsh. 



Formation and Locality. — Coal Measures ; Mazou Creek, Illinois. 



Acanthodes beecheri, sp. nov. 



Text-figure 14. 



A very small species, attaining an extreme length of about 5.5 cm. Body 

 elongated and slender, the maximum depth being contained about nhie times 



1 Reis, 0. M., Ueber Acanthodes bronni, Agassiz. Morphol. Arb., Vol. VI. 

 (189G). Plate VI., Fig. 11. 



2 Fritsch, A., Fauna der Gaskohle, Vol. III. (1893), p. 71. 



3 Jaekel, 0., Ueber die priraare Zusammensetzung des Kieferbogens and Schul- 

 tergiirtels. Verhandl. deutsch zool. Ges. (1899), p. 256, text-fig. 2. — On tlie micro- 

 scopic structure of Acantliodian scales, see the articles by lieis already cited, and 

 Rolion's Memoir on Die Obersilurischen Fische von Oesel (Mem. Acad. Imp. ScL 

 St. Petersburg, Vol. XLI., 1893, No. 5, p. 22). 



