172 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



long as the latter and almost as well developed. It is composed of from 8 to 21, 

 usually 12-15, segments, and is from 2 to 8, usually 4-6, mm. long. P 2 is the longest and 

 stoutest pinnule. It is composed of from 8 to 21 segments, the most common num- 

 bers being 1 1-13 and 15-18. It tapers gradually and evenly from the base to a delicate 

 tip, is more or less recurved, and its segments become more or less elongated distally. 

 It is from 3 to 13, most commonly 6-9, mm. long. P 3 varies from 1.5 to 8 mm. in 

 length, being usually either about 3 or 6-7 mm. long. It is composed of 8-15, 

 usually either about 8 or about 14, segments. It may closely resemble P 2 , being only 

 slightly smaller, or it may resemble P 4 and the pinnules following; but it is usually 

 intermediate between P 2 and P 4 . 



The segments of the proximal pinnules beyond the basal become more or less 

 elongated with usually more or less prominently spinous distal ends; they are rarely 

 entirely smooth, or practically so. 



On the basis of our present knowledge specific lines within the genus Decametra are 

 difficult to draw, and the present interpretation of specific limits must be regarded as 

 largely tentative. 



Decametra intermedia is evidently based upon an immature individual, and is 

 probably the young of one of the other species. In mature individuals of species of 

 this genus P! is never longer than P 2 . 



Decametra zebra from the coast of New South Wales is rather sharply separated 

 from the other species of the genus by the spinous carination of the basal segments 

 of P 2 . 



Decametra chadwicki from the Gulf of Suez and D. tigrina from southern Japan 

 form a well-marked group characterized by the possession of 18-26 cirrus segments, 

 most of which are about as long as broad, by having the lower pinnules markedly 

 stiffened and composed of segments with prominently spiny distal ends, and by 

 having P 2 about as long as the cirri and composed of about 20 segments. 



Decametra alaudae, D. taprobanes, and D. arabica agree in having a large number 

 (23-29) of cirrus segments, in having P 2 much longer than, often nearly or quite twice 

 as long as, P] and composed of 15-18 segments, and in having P 3 similar to P 2 though 

 smaller. As these three forms are all from the Indian Ocean, ranging from Muscat 

 southward to Ceylon and Cargados Carajos, it is possible that they represent in 

 reality the same specific type, or more or less minor variations of the same species. 



Decametra laempinna, D. studeri, D. informis, and D. modica are rather sharply 

 distinguished from the remaining species through having the dorsal transverse ridge 

 on the cirrus segments either continued as far as the antepenultimate segment 

 (laevipinna) or narrowing to a single median tubercle or spine on from 4 to 6 seg- 

 ments preceding the penultimate; laempinna appears to be a very distinct species, and 

 studeri is probably a good species; but it is quite possible that informis from the 

 Malayan region and modica from the Indian Ocean may turn out to be at most minor 

 variations of the same specific type. 



Of the remaining species, mylitta and mollis have 20-25 cirrus segments, P 2 with 

 14-17 (usually 16-17) segments, and the arms 55-75 mm. long. There is little real 

 difference between mylitta from the Malayan region and mollis from the eastern 

 Indian Ocean, and it is quite possible that they represent hi reality the same specific 

 type. 



