230 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



can be everted or closed down over the grooves. At the same time he figured the 

 highly developed side and covering plates in Paicilometra acosla (fig. 1169, pi. 27), 

 Perissometra angusticalyfc, Aglaonietra incerta (fig. 1166, pi. 27), Pachylometra 

 incequalis (fig. 1168, pi. 27), and GJiaritometra basicurva (fig. 1167. pi. 27), species 

 collected by the Challenger and described in 1888. 



In 1886 Carpenter found highly developed side and covering plates in an 

 arctic species which he called Antedon brtrentsi, and which since has proved to be 

 an inconstant variation of Heliometra glacialis. 



In his memoir on the comatulids of the Challenger expedition published in 

 1888 Carpenter used the occurrence of highly developed side and covering plates 

 as the chief criterion for systematic differentiation in the multibrachiate endocyclic 

 types and as a secondary character of the highest importance in the 10-armed 

 endocyclic forms. But, excepting in those species in which they are very highly 

 developed, belonging to the families Calometridse, Thalassometridae, and Charito- 

 metridse, he does not mention the ambulacral armament at all, and even in them 

 his figures and descriptions appear to have been based upon dried material, there 

 being no evidence that he ever isolated and studied the plates. Carpenter denied 

 the existence of an ambulacral skeleton in the Comasteridae. 



Dr. Clemens Hartlaub in 1895 described in detail and figured the side and 

 covering plates in a new species of Thalassometra, Th. agassizii (figs. 1171, 1172, 

 pi. 27). The figure, drawn from material treated with hot caustic potash, shows 

 large side plates parallel with the borders of the pinnulars, alternating with 

 small side plates at right angles to these which bear the covering plates. 



Hartlaub suggested a new division of the species of the genus Antedon as 

 then understood, placing all the forms with " plated ambulacra " in one series, 

 which he called Series I, and all those with " unplated ambulacra " in another 

 series. Series II. But in his Series I he puts Carpenter's " Elegans Group" (the 

 genus Zygometra) , in which the ambulacra are " unplated " and also Carpenter's 

 "Basicurva Group," which, as outlined by Carpenter, contains species with 

 " unplated " as well as with " plated " ambulacra. Similarly, in his Series II he 

 includes the " Eschrichti Group" of Carpenter (Heliometra, Solanometra, and 

 Florometra) containing species which occasionally have the ambulacra heavily 

 plated. 



In 1902, Mr. Frank Springer described a new comasterid, Nemaster iowensis, 

 from Florida, in which the ambulacra are bordered with highly developed covering 

 plates, a feature not hitherto noticed in the family. 



Dr. Th. Mortensen in 1903 described and figured large and well-formed ambu- 

 lacral plates in a specimen of Heliometra glacialis from East Greenland, as well 

 as the ambulacral rods of Hathrometra prolixa. In 1910 he described and figured 

 another type of ambulacral rod in Hathrometra prolixa, as well as the spicules in 

 the tentacles ; and in 1918 he described the side and covering plates and the spicules 

 in the tentacles in Notocrinus ririlis (figs. 1329, 1330, pi. 49) and Isometra vivipara 

 (figs. 1328, 1331, pi. 49) and determined the complete absence of both types of 

 deposits in Thaumatometra nittrix. 



The description of the ambulacral armature and of the spiculation of the 

 tentacles in a representative series of comatulids follows : 



