Lyman.] 106 [March 7, 



On comparing the distance between the forks of the three species, 

 or, in other words, the lengths of the shafts, it will be seen that 

 although they branch according to the same rule, they follow three 

 types or patterns, which depend on the relative lengths of the shafts 

 or parts of the arm lying between the forks. (See Table in.) 1 A. cos- 

 tosum begins with shafts of 9 mm., which diminish to 5 mm. at the 

 ninth fork, and to 3 mm. at the sixteenth, which number continues 

 to the end, or twenty-sixth fork, and the average length of the shafts 

 for the half-arm is only 4.7 mm. A. Agasizii begins with shafts of 

 27 mm., which diminish to 14 mm. at the eighth fork, and to 8 mm. 

 at the last fork (twelfth). The average is 16.4 mm.", or nearly four 

 times that of A. costosum. A. asperum beginning with 12 mm., a 

 shaft longer than that of A. costosum, suddenly diminishes to 4 mm. 

 at the seventh fork, then to 2 mm. at the ninth, and 1 mm. at the 

 twelfth, which is constant to the end (eighteenth). The general 

 average is only 3.5 mm., considerably less than in A. costosum. To 

 summarize the three types, we may call that of A. costosum a branch- 

 ing, with numerous short, nearly equal shafts' (Plate 7, fig. 2) ; of 

 Agassizii, few, long, unequal shafts (Plate 7, fig. 3) ; of A. asperum, 

 numerous short, suddenly diminishing shafts (Fig. 4). 



These three types of branching correspond to as many differences 

 of structure and locality. A. costosum makes a group with A. sjrino- 

 sum and A. cacaoticum, characterized by an arm very broad at its 

 base, and supported by a small disk with re-entering curves. The 

 habitat is in the warm waters on either side of Central America. 

 The second group includes all the species of the colder seas, such as 

 A. Agassizii, A. eucnemis, A. Stimpsonii, and A. Pourtalesii. The 

 disk is rounded and large, and the arms narrow at their origin. 

 A. asperum inhabits the Indian Ocean, and constitutes by itself 

 the third group, well marked by its short terminal twigs, which give 

 a look like moss to the periphery of the animal. 



One characteristic observable among Astrophytidae is the exag- 

 gerated development of the arms. Astrogomphus among the simple- 

 armed Astrophytons shows proportionately the widest disk, whose 

 diameter goes six times into the length of the arm. Astroschema has 

 an arm more than thirty times the disk. 



But these proportions are trifling compared with those of Astro- 

 pliyton itself. If the lengths of all the branches and twigs of an arm 



iThe numbers in Table in, compared with corresponding ones in Tables I and II, 

 differ a Uttle, because different stems were measured. 



