A MONOGRAPH OF THE EXISTING CRINOIDS 29 



the comatulids very young individuals have the ligamentary connections in the form 

 of synostoses. The distal face of the hypozygal is somewhat concave, and the proxi- 

 mal end of the epizygal somewhat convex. The change from synostosis to another 

 form of ligamentary union seems to take the following course. The synostosis becomes 

 less close— in other words, the ligamentous fibers between the ossicles in certain places 

 become longer. Sometimes the contact is retained about the lumen, resulting in an 

 areola, and the contact seems to be closer at the margin of the ossicles. If the articu- 

 lation is to develop into a synarthry, a strong areola always appears from which a 

 ventral and dorsal beam grow out, resulting in the dorsoventral articular ridge. But 

 if the articulation is to develop into a syzygy an areola may appear and persist, or 

 it may be altogether lacking. The septa, very feeble at first, appear in the dorsal 

 and lateral parts of the joint faces and are extended outward, because with the apposi- 

 tional increase of growth in the articulation they continue in contact with the margin. 

 During the development the septa increase in number, and the new ones that are 

 formed later arise in the same way from the border of the joint face, intercalated 

 between the septa first formed, which extend into the neighborhood of the central 

 lumen. That the septa must also to some extent increase in growth toward the center 

 is seen by the increasing number of complete septa in older invididuals. It is cer- 

 tainly true that the lumen is somewhat enlarged during growth, but this is not suffi- 

 cient to explain the increase of complete septa. In a young Antedon petasus the 

 diameter of the ossicle is 0.31 mm., and of the central lumen 0.06 mm. There are 

 two complete septa. In a full grown individual the lumen is 0.19 mm. in diameter, 

 and there are 12 complete septa. 



The variation in the number of septa in any given species at any given age is 

 rather small and unimportant. In a Mariametra subcarinata, for example, Gislen 

 found the number of septa on the distal end of four third brachials to be 15-16. In 

 a specimen of Heliometra glacialis the number of septa on the distal end of the third 

 brachial was 36-36, and in another of Promachocrinus kerguelensis the number was 

 27-30. The type of syzygy was here, as everywhere else, uniform within the species. 

 In strongly multibrachiate forms the size of the segment and the number of septa 

 may vary a little more, according to whether one examines an inner or an outer arm, 

 and the number of axillaries preceding the arm, as for instance in Capillaster, Coman- 

 theria, and Stylometra. Beyond the first syzygy the number of septa in a few syzygies 

 may increase slightly, but fairly soon the septa decrease in number, as a natural result 

 of the distal portion of the arms being younger and slenderer than the proximal. The 

 ninety-first brachial (about the thirtieth hypozygal) in a fully grown Heliometra 

 glacialis with arms about 200 mm. long had only 22 septa, consisting of 7 ven- 

 tral rows of tubercles and 15 more or less incomplete septa. The hypozygal follow- 

 ing had the same number of septa. In the small Isometra vivipara with arms about 

 60 mm. long the distal face of the third brachial had 14 septa. In this specimen the 

 fourth or fifth syzygyial articulation was developed as a synostosis, although not a 

 close one, and lacked septa. 



Large and stout types have usually an unusually large number of septa for the 

 systematic units to which they belong, while small and more delicate forms have an 

 unusually low number. Also young and immature individuals have fewer septa 

 than fully grown individuals of the same species. A young Heliometra glacialis from 



