ASTEROLEPIS MAXIMA. 87 



The Pectoral Appendages or Arms. — It, is largely through the study of casts and 

 impressions like those figured in PL XVIII, figs. 1 — 3, that we attain to 

 a knowledge of the structure of the arm in this species, as the bone structure is 

 extremely friable, and has an obstinate tendency to adhere to the matrix rather 

 with its external sculptured than with its internal smooth surface. However, the 

 knowledge gained from such casts and impressions added to that afforded by 

 others, like the portion of arm seen in PI. XVIII, fig. 4, in which the bone is 

 more or less retained, shows very clearly that in the configuration of its parts the 

 pectoral limb of Asterolepis maxima is as near as possible to that of A. ornata. 



The general form and structure of the Asterolepid arm, and the nomenclature 

 of its plates having been already given at p. 68, it now remains to describe 

 specially the parts as seen in the present species. 



Upper Arm. — Each articular plate (ar., woodcuts, Figs. 37 and 38) is rounded 

 proximally and internally to form part of the articular " head " of the limb, while 

 distally it is pointed. The two plates are related to each other and to the brachial 

 process of the anterior ventro-lateral in the manner described at p. 68. As to 

 their relations to other plates, there is some difference between the dorsal and the 

 ventral articular. The former (d. ar.), which is longer than its fellow, articulates 

 by its outer edge with the external marginal ; by its truncated apex with the dorsal 

 anconeal; by its inner margin with the internal articular and the internal marginal. 

 The ventral articular (v.ar.) is likewise joined externally to the external marginal, 

 but its apex being pointed its inner side articulates with the ventral anconeal and 

 the internal articular, so that it does not touch the internal marginal at all. 



The internal articular (i. ar.) is situated entirely on the inner aspect of the arm,, 

 and is therefore not seen either from above or below except in casts, where its 

 impression is obliquely visible from the ventral aspect (PI. XVIII, fig. 3). Its 

 form is well seen on the portion of arm represented in fig. 6 of the same plate. 



Its free upper margin is concavely excavated to form a shallow notch over 

 which the nerves and nutrient vessels must have passed from the brachial foramen 

 into the arm ; its dorsal margin is obtusely angulated, and articulates with the 

 dorsal articular and the internal marginal ; its pointed apex is wedged in between 

 the last-named plate and the ventral anconeal ; its ventral margin, which is gently 

 convex, articulates proximally with the ventral articular, distally with the ventral 

 anconeal. 



The external marginal (e. m.) extends the whole length of the outer side of the- 

 upper arm. It is narrow above and below, broad in the middle, and longitudinally 

 folded upon itself along the margin of the limb, but not symmetrically, as the 

 ventral portion is broader than the dorsal. Its pointed proximal extremity 

 completes the slit between the two articulars externally (PI. XVIII, fig. 5) ; its 

 broader lower extremity receives within it the articular process of the upper 



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