18 BULLETIN OF THE 



The median row of plates is well marked, even into stages of the star- 

 fish of some size, and each plate bears at first a single spine.* 



Lateral Dorsal Plates. — • The larger members of the network of plates 

 which connect the median dorsals with the marginals may be known as 

 the lateral dorsal plates. They were first detected in a young starfish 

 in which were four median dorsal plates ; and the first pair to ap- 

 pear is situated on one side of the third median dorsal. In a specimen 

 older than the last these plates were found on the second, third, and 

 fourth median dorsal plates. It will thus be seen that the first lateral 

 dorsal to form is not the pair which belongs to the first, but to the third, 

 median dorsal. 



The lateral dorsals (dl) are semicircular or circular plates, with their 

 longer axes at right angles to the line of the radius. They are in young 

 stages destitute of spines. There seems to be little regularity in the 

 formation of additional lateral dorsals, and in older conditions they form 

 a dsedalus of plates very difficult to trace. They are intimately connected 

 by smaller calcifications, which will be spoken of as the connectives. 



Genitals.f — The genitals {g^—g^) are the first interradial plates to form. 

 These plates are among the earliest plates of the starfish brachiolaria, 

 and in early stages in the growth of the body they are very conspicu- 

 ous. They probably originate after the terminals, and appear at first as 

 small calcareous nodules, alternating with the terminals. All of the 

 genitals, in young stages, are smaller than the terminals. The fact that 

 thev originate after the terminals is not an imimportant one. as it shows 

 that in this particular the starfish resembles Amphiura. Moreover, it 

 has been stated that one (^g^) of the genitals — namely, that near the 

 madreporic body — arises before the terminals. I find this statement, 

 as well as another that the size of the " basal " (genital) near the madre- 

 poric opening is larger than the remainder, and preserves its preponder- 

 ance in size in all younger stages of the growth of the starfish, not to 

 hold in the specimens of Asterias which were examined. 



The genitals when first formed are simple nodules, which later form 

 branched spicules, as shown in Plate I. fig. 1. They lie in the interval 

 of the interradii between the terminals, while their centre of calcification 

 always begins in an interradius. 



* The median row of dorsals and their spines correspond witli the " median 

 line of spines supported by a long narrow limestone plate extending from the basal 

 plate almost to the terminal radial," mentioned by A. Agassiz (p. 51, op. cit.). 



t The same plates as those called in several late writings the basals, from 

 their supposed homology with the basals of Crinoids. 



