134 OPHIUROIDEA. 



Ophiura to some of the species placed by Miiller and Troschel in their genus 

 Ophiolepis. 



Many new species have been discovered and described by Sars, Liitkin, Gnibe, and 

 Phihpi, and a few new genera to include the same have been proposed by them as 

 Ophioplus, Sars, which also includes Amphiura, Forbes, Ophiocten, and Ophiactis, Liitkin, 

 besides OpJnopholis and Ophiopeza, Peters. 



The fossil species belong to existing genera, as Ophioderma, Ophiolepis, Amphiura, 

 Ophiocotna ; and to extinct genera, as Ophiurella, Acroura, Aspidura, Geocoma, Palaocoma, 

 Aplocoma, and Protaster. (See Plate XIV.) 



On the Skeleton and tegumentary Framework of the Ophiuroidea. 



The framework of the Ophiurida consists of a calcareous or internal, and a tegu- 

 mentary or external, skeleton. The former is composed of a number of calcareous osselets 

 of the uniformly reticulate structure common to all the Echinodermata. These pieces are 

 variously arranged in the different genera. The disk, in this order, forms the entire 

 visceral cavity, no prolongation of the same extending into the arms, as in the Asteriada ; 

 on the under surface of the disk five pairs of bones occupy the interbrachial spaces, and 

 form a series of arches that afford attachment to the delicate osselets constituting the 

 skeleton of the upper surface; they are likewise covered with large plates carrying 

 conical-pointed processes, with short spines, which project inwards towards the mouth, 

 and perform the part of jaws and teeth. Several forms of these dental organs are seen 

 in PI. XIII, figs. 1, 4, 5, 6. 



Prom the interbrachial spaces five long arms proceed ; these organs are supported 

 on a series of osselets resembling the bodies of caudal vertebrae in the tails of lizards ; the 

 scaly character of the tegumentary membrane covering the same increases the resemblance 

 between the ray of Ophiura texturata, Lamk., and a lizard's tail. The brachial osselets 

 have on their anterior and posterior surfaces transverse processes or kinds of articular 

 condyles directed in opposite directions, the one vertical, the other horizontal; two of these 

 transverse condyles always cross each other, so that the lateral flexion of the arms is in 

 nowise impeded ; these pieces carry likewise discoidal expansions more or less flexed, 

 which form spaces to receive the base of the fleshy tentacles near the disk. 



To the internal skeleton likewise appertain the elongated processes found in the visceral 

 cavity at the base of the arms in relation to the reproductive organs, and the many 

 small osselets arranged on the border of each of the five fissures leading from the mouth- 

 opening to the centre of the base of the arms (PI. XIII, figs. 4, 5, 6). Belonging to 

 the same class are the many small pieces piled upon each other at the extremity of the 

 interbrachial bones, and forming the five projecting cones or jaws surrounding the mouth, 

 and which perform the office of teeth (PL XIII, fig. 6). 



