1889.] NATURAL SCIENCES OF PHILADELPHIA. 77 



In these structural characters they differ from 0. teres in which 

 the upper arm-plates are broken into two or more pieces, the radial 

 shields covered and the mouth shields usually broad and short with 

 a nearly straight outer edge, concave sides and a rounded lobe pro- 

 jecting inwards. The arm sjnnes usually number nine in specimens 

 corresponding in size to those of 0. Panamensis in which eleven are 

 found. 



One of the specimens loaned by the Museum of Comparative 

 Zoology and labeled " Gulf of California," agrees in general char- 

 acter with 0. teres, but possesses uncovered radial shields. There is 

 also a very young (four-armed) alcoholic specimen in the collection 

 of the Academy in which the radial shields are naked. Mr. Lyman 

 has described the young of 0. teres as having uncovered radial 

 shields. Prof. Verrill, however, states that this is not the case with 

 the young forms in the museum of Yale College, and I find that the 

 young specimens of 0. teres, in the collection of the Academy, with 

 the single exception of the specimen above mentioned, have the 

 radial shields granulated as in the adult. These two specimens and 

 those mentioned by Mr. Lyman (Proc. Bost. Soc, vii, p. 257) seem 

 to indicate, nevertheless, that the young occasionally have the radial 

 shields uncovered. 



The color of the upper surface of the disk and arms of 0. teres is 

 of various shades of brown, sometimes speckled with grayish white. 

 In an alcoholic specimen from Nicaragua the arms are very irreg- 

 ularly banded with light brown, the outer and inner plates of a 

 dark band being darker than the other plates forming the band. 



The darkest variety of 0. teres closely resembles the darkest 

 variety of 0. Panamensis. In both forms some of the upper arm- 

 plates of the outer portion of the arm have white markings, which 

 form segments of two broken parallel longitudinal lines, one on 

 each side of the arm. Each segment occupies one or more plates and 

 is separated from the next segment by an also variable number of 

 plates without any such markings. 



It will be seen from the above that 0. Panamensis presents a very 

 great variety of color pattern. This variation is probably due to 

 the wide north and south range of the species, viz : from Panama to 

 California. It is noticeable that the darker varieties are found 

 northwards, the darkest specimens in the collection of the Academy 

 coming from San Diego. 



