430 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. loi 



Additional distribution record. — Panama. 



Remarks. — Whether this species is identical with C. decorata 

 (Drayton) from Honden Island is uncertain. According to Verrill, 

 both species agree in color. 



Family METRIDIIDAE ? 



ISOMETRIDIUM, new genus 



Metridiidae ? with broad base. Column divisible into a smooth, 

 very thick scapus, wrinkled in contraction, and a thinner capi- 

 tulum. No cinclides. Sphincter mesogloeal, situated in the upper- 

 most part of the scapus and in the capitulum. Uppermost part of 

 the capitulum and outer part of the oral disc drawn out to form 

 very numerous, permanent lobes. At the apex of these lobes are 

 extraordinarily numerous, short, filiform tentacles, all of about the 

 same length, their longitudinal muscles ectodermal. Oral disc 

 forming a wall inside the tentacles. Radial muscles well developed, 

 ectodermal, present only on the inside of the lobes and on the out- 

 side of the wall ; inner part of oral disc, however, without radial 

 muscles. Two very broad siphonoglyphs. Mesenteries very nu- 

 merous, several pairs perfect, two pairs of directives; the 12 

 first pairs sterile. Retractors of the mesenteries diffuse, not 

 strong. Acontia numerous. Cnidom : spirocysts, basitrichs, micro- 

 basic ?)-mastigophors, microbasic amastigophors (and possibly 

 microbasic 6-mastigophors). 



The position of this genus is doubtful because I cannot decide 

 the type of the nematocysts of the acontia, especially whether there 

 are basitrichs or &-mastigophors present. If the acontia have basi- 

 trichs, the genus may be referred to the family Sagartiidae, but in 

 such a case Isometridium would be a very aberrant genus in that 

 family. The thick column recalls that of many genera of the family 

 Actinostolidae. 



ISOMETRIDIUM RICKETTSI, new species 



Figure 81, a, b; Plate 14, Figures 3, 4 



Scapus up to almost 1 cm. thick in preserved state. Sphincter of 

 the scapus transversely stratified, that of the capitulum forming a 

 single row of muscle meshes. Tentacles very delicate, not arranged 

 in distinct cycles. Oral disc inside the wall without radial muscles. 

 The very broad siphonoglyphs provided with very large gonidial 

 tubercles. Mesenteries very numerous ; at least the stronger hex- 

 amerously arranged. Probably considerably more mesenteries dis- 

 tally than proximally. About 48 pairs of mesenteries perfect, the 

 12 oldest pairs sterile, the others perfect and the stronger im- 

 perfect fertile. Nematocysts of the capitulum partly 26.8-35.2 by 

 5-6.6 IX (basitrichs), partly 19.7-24 by 3.5-4.2 fi (microbasic 



