MR. P. H. CARPENTEE ON THE GENUS ACTINOMETRA. 443 



distinctive character between Antedon and Actinometra. The dif- 

 ferences in the position of the mouth, however, afford a distinctive 

 character of much value, especially as they are accompanied by 

 marked variations in the anatomy and mutual relations of impor- 

 tant internal organs. 



Out of the numerous species of Gomatula at present known, the 

 author has been able to refer seventeen to the genus Actinometra 

 as above defined. Fourteen of these were known to Miiller ; and 

 out of the remaining twenty-three species described by him, the 

 author has determined sixteen as true Antedons with a subcentral 

 mouth, together with nine species described by various authors 

 since the publication of Miiller's memoir. Seven of Miiller's spe- 

 cies remain as yet undetermined, together with one described by 

 Pourtales, as there is no mention of the position of the mouth in 

 the specific diagnoses, and the author has been unable to make a 

 personal examination of the specimens in question. 



Taking, therefore, the genus Actinometra as including all those 

 Comatulce in which the mouth is excentric, the author finds that 

 it may be divided into two principal groups according to the 

 position of the mouth with respect to the radial or ambulacral 

 planes. In Antedon, where the mouth is subcentral, the interradial 

 area containing the anal tube may be considered as posterior ; and 

 a plane passing through the mouth and anus so as to divide the 

 disk into two symmetrical halves, will traverse the odd ambulacrum 

 in front of the mouth, which may therefore be regarded as radial 

 in position. 



In Actinometra Solaris the odd ambulacrum is also in front of 

 the mouth, which, though excentric in position, lies in the radial 

 half of a plane passing through the mouth and anus, so as to divide 

 the disk into two symmetrical halves, the interradial half of this 

 plane passing through the anus. 



In Act. multiradiata, however, and in many other species, the case 

 is different, as the mouth is interradial in position, and the odd 

 ambulacrum lies behind it ; for a plane cutting the mouth and 

 anus is radial behind the mouth, in front of which it passes along 

 the interval between two ambulacra or radii. 



This type of Actinometra, in which the mouth is interradial and 

 the odd ambulacrum lies behind it, is considerably more frequent, 

 so far as the author's experience goes, than the simpler type, in 

 which the mouth is radial and the odd ambulacrum anterior, as in 



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