54 



BULLETIN 110, UNITED STATES NATIONAL MUSEUM. 



With specimen No. 8367, U.S.N.M., are a. considerable number of parts of the 

 abdominal ribs, several of which are in a beautiful state of preservation, as shown 

 in figure 38. While these were found disassociated, a study of them in conjunction 

 with those of specimen No. 4734, U.S.N.M., enables me to determine with some de- 

 gree of certainty several points hitherto obscure. The more characteristic of these 

 segments are shown in figure 38. 



The long slender bone shown in A, (fig. 38) is unquestionably the outer or most 

 lateral segment of an abdominal rib. Its outer half is rounded in section and sinu- 

 ous in its curvature, with a tapering ex- 

 tremity. The inner or proximal half is 

 flattened on one side, probably the an- 

 terior, along which a similar, though 

 oppositely flattened side of the second 

 segment laps (fig. 39 and pi. 15), where 

 they are shown articulated as found in 

 place. 



The second segment (D and E, fig. 38) 

 is a regularly curved bone deeper than 

 wide, with a tapering outer extremity, 

 and a slightly enlarged proximal articu- 

 lating end, the face of which is oblique 

 to the longer axis of the bone. The 

 posterior face of this end of the bone is 

 flat, but gradually changes to a longi- 

 tudinal groove as the outer end is ap- 

 proached. The articular end articu- 

 lates with the third segment as shown 

 in E, figure 38. 



The third segment (B, and lower 

 part of E, fig. 38) is short with a very 

 much expanded distal end having a cup- 



Fig. 38. — Segments of abdominal ribs of Antrodemus va- , r , ° 



lens leidy. no. 8367, tj.s.n.m. a, lateral segment; b, ped articular surf ace that is oblique to 



THIRD SEGMENT WITH ARTICULAR PROXIMAL END; C, SHORT ^Q l on p. er ax ig Q f the bone. It is tMs 



SEGMENT POSITION UNDETERMINED; D, SECOND SEGMENT SHOW- & .. 



mG the expanded distal articular end; e, second seg- expanded, cupped end that articulates 



MENT, SAME AS D, BUT COOSSIFIED BY OBLIQUE TRANSVERSE ^.^ ^ pr0X i ma l en( [ f the SCCOnd 



JOLNT WITH THIRD SEGMENT. ALL FIGURES -J NAT. SIZE. .,, i . -i-i ^ nn 



segment as illustrated .in E, figure 38. 

 The opposite end, as rather unsatisfactorily shown by two bones, appears to be grooved 

 on the superior side for the articulation of a fourth segment, and in specimen No. 4734. 

 U.S.N.M. (pi. 15) a series of four segments are to be observed in the third rib from the 

 bottom of the block. It may be that the unidentified segment shown in C, figure 38, 

 represents the fourth of the series. Whether this fourth segment joined the one of the 

 opposite side at the median line, or whether there was a median bone that joined 

 the two series, no explanation is afforded by the materials now before me. There 

 is no evidence of a bilateral median segment among the many abdominal parts 

 preserved with the two specimens except the one shown in figure 37, and this appears 



