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 8cm., has a width of 8mm., 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 
stiffened, 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 outline 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 ; Mazon Creek, Illinois. 
Acanthodes beecheri, sp. nov. 
Text-figure 14. 
A very small species, attaining an extreme length of about 5.6 cm. Body 
elongated and slender, the maximum depth being contained about nine times 
1 Reis, O. M., Ueber Acanthodes bronni, Agassiz. Morphol. Arb., Vol. VI. 
(1896), Plate VI., Fig. 11. 
2 Fritsch, A., Fauna der Gaskohle, Vol. III. (1893), p. 71. 
3 Jackel, O., Ueber die primare Zusammensetzung des Kieferbogens und Schul- 
tergiirtels. Verhandl. deutsch zool. Ges. (1899), p. 256, text-fig. 2.— On the micro- 
scopic structure of Acanthodian scales, see the articles by Reis already cited, and 
Rohon’s Memoir on Die Obersilurischen Fische von Oesel (Mém. Acad. Imp. Sci. 
St. Petersburg, Vol. XLI., 1893, No. 5, p. 22). 
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