92 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



ence by scars, but there must have been at least one pair of strong muscles extending from 

 the under side of the head across to the hypostoma. Judging from the method of attach- 

 ment, the muscles moving the limbs were short ones, chiefly within the segments of the legs 

 themselves. 



Flexor Muscles. 



Since the majority of trilobites had the power of enrollment, and seem also to have used 

 the pygidia in swimming, the flexors must have been important muscles. Beecher (1902, 

 p. 170) appears to have been the only writer to point out any tangible evidence of their 

 former presence. Walcott (1881, p. 199) had shown that the ventral membrane was 

 reinforced in each segment by a slightly thickened transverse arch. Beecher showed that 

 on this thickened arch in Triarthrus, Isotelus, Ptychoparia, and Calymene, there are low lon- 

 gitudinal internal ridges or folds. One of these is central, and there is a pair of diagonal 

 ridges on either side. Beecher interpreted these ridges as separating the strands of the 

 flexor muscles, and believed that a line of median ridges divided a pair of longitudinal 

 muscles, while the outer ridges showed the place of attachment of the pair of strands which 

 was set off to each segment. He did not discuss the question as to where the anterior and 

 posterior ends were attached. In trilobites with short pygidia, the attachment would prob- 

 ably have been near the posterior end, and it is possible that the two scars beneath the dou- 

 blure and back of the last appendifers in Ceraurus may indicate the point of attachment in 

 that genus. 



There is as yet no satisfactory evidence as to where the anterior ends of the flexors 

 were attached. In Apus these muscles unite in an entosternal sinewy mass above the mouth, 

 but no evidence of any similar mass has been found in the trilobites and it is likely that 

 the muscles were anchored somewhere on the test of the head. 



Extensor Muscles. 



The exact position of these muscles has not been previously discussed. The interior of 

 the dorsal test shows no such apodemes as are found on the mesosternites, but, as I have 

 shown in the discussion of the alimentary canal of Calymene and Ceraurus, there is an 

 opening on either side of the axial lobe between the dorsal test and the abdominal sheath, 

 and it is entirely probable that an extensor muscle passed through each of these. The ab- 

 dominal sheath extends only along the posterior region of the glabella and the anterior 

 part of the thorax, and probably served to protect the soft organs from the strain of the 

 heavy muscles. The extensors (see fig. 29) probably lay along the top of the axial lobe 

 on either side of the median line of the thorax to the pygidium, where they appear to 

 have been attached chiefly on the under side of the anterior ring of the axial lobe, although 

 strands probably continued further back. This is above and slightly in front of the fulcral 

 points on the pleura, and meets the mechanical requirements. Ceraurus (Walcott, 1875, and 

 1881, p. 222, pi. 4, fig. 5) shows a pair of very distinct scars on the under side of the first 

 ring of the pygidium, and in many other trilobites (Illatnus, Goldius, etc.) distinct traces of 

 muscular attachment can be seen in this region, even from the exterior. The anterior ends 

 were probably attached by numerous small strands to the top of the glabella, and, principally, 

 to the neck-ring. 



On enrolling, the sternites of all segments are pulled forward and the tergites backward. 

 In straightening out, the reverse process takes place. The areas available for muscular at- 



