REPORT ON THE CRLNOIDEA. 277 



but as it never occurs in Antedon, it affords another good potential character of the genus 

 Aetinometra. 



The distribution of the genus, both in space and in time, has been discussed already 

 on pp. 35-40. 



Classification. — The species of Aetinometra, like those of Antedon, fall into certain 

 very well defined series, as shown above on pp. 57-59. But the various series do not 

 altogether correspond in the two genera. Each has a ten-armed, a bidistichate, and a 

 tridistichate series. But the first two of these are epiite small in Aetinometra, although 

 they together contain three-fourths of the described species of Antedon. On the other 

 hand, the tridistichate series of the former genus is remarkable for the number and 

 variety of the forms which it includes, though it is poorly developed in Antedon. There 

 are, however, three tridistichate species of Antedon with a considerable number of arms ; 

 but in all three alike the two outer radials are united by syzygy, and each arm-division 

 above the distichal axillary consists of two or three joints, in the latter case with a 

 syzygy in the axillary (Pis. VIII., IX.; PI. XXXVII. fig. 4). This combination of 

 characters does not present itself in any Aetinometra yet known. In all the four 

 tridistichate species of this genus which have the two outer radials united by syzygy, 

 each arm-division above the distichal axillary consists of two joints, which are themselves 

 united by syzygy, as the radials are (PI. LV. fig. 1 ; PI. LVI. fig. 3 ; PI. LVII. fig. 1). 

 In another group the two outer radials and the first two distichals, palmare, and 

 brachials are respectively united by syzygy (PI. LIV. figs. 1,2); while in a third the 

 radials and brachials have the same characters, but distichals are undeveloped, so that 

 there are only ten arms (PI. LIII. figs. 2, 15). Neither of these three types is represented 

 in the genus Antedon at all, and they are all strictly limited to the Eastern Archipelago, 

 ranging from Mergui on the west to Fiji on the east, but scarcely passing the limits of 

 the tropics. Neither of them is at all rich in species, though the range of variation 

 within the limits of a single specific type is in some cases very considerable. They may 

 be classified as follows : — 



Series I. 

 The two outer radials, and the first two brachials respectively united by syzygy. 



Ten arms, . . . . . • • ■ • • • 1- Solaris. 



Two distichals united by syzygy, . . . • • • • -2. Paucicirra. 



Three distichals, the axillary a syzygy, . . • • • ■ ■ 3- Typica. 



