MONOGRAPH OF THE EXISTING GRINOIDS. 



211 



Heterometra crenulata and H. phUiberti among the oligophreate. They are 

 found, rather less developed, in Perometra (part 1, fig. 112, p. 179), Asterometra 

 (fig. 209, p. 149, and part 1. fi<r. 94, ]>. 155), and Ftilometra^ and in certain other 

 of the larger and stouter species of many other genera. 



In many oligophreate species specimens are often found with a white or very 

 light mediodorsal line along the division series and the arms, this line being 

 usually bordered on either side by a narrower dark or black line. Very fre- 

 quently this line resembles a mediodorsal carination so closely as to be with 

 difficulty distinguished from it, and the confusion is increased by the fact that 

 both the line and the carination very often, as in certain species of Comatula^ 

 occur together. Such a dorsal stripe is invariably found in the species of the 

 genera Eudiocrinus and Cotylometra and usually in the species of Colohometra and 

 Pterometra. It is very common in all of the species of Comatvla and in many 

 of those of Co7nanthus and Comanthina, as well as in various species of Cyllo- 

 inetra and Pontiomefra, and in OUgometra serripinna. 



A somewhat less common condition is a narrow black mediodorsal line. Such a 

 line is seen in Coccometra nigrolineata, in Mariametra margaritifera, and in many 

 of the species of Comaster, in all of which it is fairly constant. Only in Maria- 

 metra margaritifera, however, does it cover a median carination. 



The ornamentation of the proximal ossicles by the development of spines 

 or tubercles on the dorsal surface or by the development of spines along their 

 borders indicates a primitive type and is characteristic of the young of many 

 species, of which the adults are almost or quite unornamented. Thus in Helio- 

 metra glacialis the elements of the division series and the lower brachials of 

 the young have very spinous borders which are entirely lost later in life, while 

 in many of the species of Thalassometra the young have an excessive development 

 of spines on the borders of all the proximal ossicles, which in the adults are nearly, 

 or even quite, smooth. 



Thus bj' a study of this type of ornamentation we are able to judge more or 

 less accurately of the comparative phylogenetic importance of many genera and 

 species and are able to appreciate the fact that the extremely ornate species of 

 such genera -ys the West Indian CHnometra and Stylovietra are really more primi- 

 tive than the corresponding species of the closely related, but far less ornate, 

 species of the East Indian genera Crossometra, Perissometra^ Monachometra, 

 Pachylometra, and Cosmiometra; and also that the spinous or tuberculous species 

 of such "enera as Glyptometra, Parametra, C osiniometra, and Thcdassometra 

 occurring in the central East Indian region are more primitive than the much 

 less spinous or tuberculous species of the same or closely allied genera occurring 

 in southern Japan, the Hawaiian Islands, New Zealand, or the Lesser Sunda 

 Islands. Similarly, it is evident that the species belonging to the genera character- 

 istic of the Antarctic region, all of which are noticeable for the development of 

 spines or keels on their ossicles, are more primitive than the species of the genera 

 characteristic of the Arctic region, all of which are smooth, though spinous when 

 young. 



