252 Field Columbian Museum — Geology, Vol. III. 



The peripherals are eleven in number exclusive of the nuchals 

 and last pygal. The series as a whole is unusually deep, especially 

 in the lateral region where the plates above the angle of the cara- 

 pace .are more than half the length of the adjacent pleurals. Only 

 the last pair is covered entirely by the marginal shields. The nuchal 

 differs from the common hexagonal type in that the anterior border 

 forms a reentrant angle at the median line. The peripheral bones 

 gradually increase in depth from first to fourth. The fourth, fifth, 

 and sixth are similar in size and are largest of the series. With the 

 seventh there is a decided reduction in antero-posterior diameter. 

 The ninth and tenth diminish rapidly in length, the eleventh is much 

 the smallest of the series. The tenth presents an equal surface to 

 articulate with the eighth pleural and the first two pygals. Peri- 

 pherals three to seven inclusive form the bridge. 



The vertebral shields are notably variable in outline. The first 

 has the form of an irregular truncated pyramid with its base at the 

 anterior marginal suture. In this specimen it is quite asymmetrical. 

 The second, third and fourth are much longer than wide. The sec- 

 ond and third are sub -rectangular in outline and nearly equal in size. 

 The fourth is somewhat smaller, narrower at the posterior end and 

 more irregular in outline. The fifth is the largest of the series. It 

 has the form of a wide triangle whose base is at the posterior mar- 

 ginal suture. 



The costal shields are four in number. The first has the form 

 of a quadrant of a circle whose center is at the mid-lateral border 

 of vertebral II. It overlies the greater part of the first and half of 

 the second pleural, as well as parts of the first four peripheral bones. 

 The second and third costals are rectangular in outline. The second 

 is somewhat broadest in its antero-posterior diameter and extends 

 over the posterior half of the second, all of the third and the anterior 

 half of the fourth pleural plates, as well as parts of the fourth, fifth 

 and sixth peripherals. The third costal shield bears a similar rela- 

 tion to the fourth, fifth and sixth pleurals, and the sixth, seventh and 

 eighth peripherals. The fourth costal shield is an irregular quadri- 

 lateral and is the smallest of the series. Its relation to the neural and 

 peripheral plates is similar to that of the second and third. The 

 intercostal sulci have the usual position opposite the middle of the 

 vertebral shields. 



The marginal shields are twenty-five in all. The sulcus separa- 

 ting them from the vertebrals and costals falls far without the pleuro- 

 peripheral suture and is little more plainly marked than those in the 



