or THE TEST OF THE TEMISTOPLETJETDJE. 349 



cess along the iuner part of the edge, bouudiug the expanded part 

 of the pit inwardly. There are knobs of reticulate tissue on alter- 

 nate projections and sockets on the others. Moreover, on one 

 projection along its broadest part is an elliptical shallow cavity, 

 and on the next a convexity occurs in the corresponding position. 

 Knobs are found, on the shelf-like process in little groups, and 

 then hollow sockets in groups, and so on. The knobs and sockets, 

 grooves and convexities of one plate-edge correspond with sockets, 

 knobs, convexities, and grooves in the edge of the opposed plate. 

 These are the sutural structures ; and they also limit the dimen- 

 sions of the pits (Plate YIII. fig. 3). 



A very similar series of structures is seen on the median angular 

 edges of the ambulaeral plates. The pits are separated by dis- 

 tinct pillars enlarging outwards, and merging inwards into a shelf- 

 like structure, on which the pit ends inwardly. The knobs and 

 sockets are on alternate pillars ; and the enlargement of the one 

 carries a long convex mass, and that of the other an elliptical 

 cavity. Usually there is a projection at the bottom of each pit, 

 from the shelf-like process. The knobs and sockets of one ambu- 

 laeral plate fit into sockets and knobs on the edge of the opposite 

 plate normally in contact, and act as sutural processes (Plate 

 VIII. fig. 8). 



Finally, there are sutures between each interradial plate and 

 the small tentaculiferous plates of the ambulacra. The small pits 

 are more or less in vertical series. On the interradial plate-edge, 

 sections of the pits are seen separated by broad processes running 

 inwards, and merging into a well- developed iuner reticulate tissue. 

 The processes and this tissue are studded with well-developed 

 sockets (Plate VIII. fig. 6). But on the corresponding edges 

 of the ambulaeral plates the similar processes and inner shelf-like 

 tissue are covered with knobs which stand out well (Plate A-^III. 

 fig. 7) ; these fit into the sockets of the corresponding interradial 

 plate, and constitute the suture. 



There is very slight and probably no absolute junction between 

 any plates by continuity of reticulate tissue ; but the suturing 

 consists of these elaborately alternate systems of knobs and 

 sockets, and of projections and corresponding grooves. 



The breadth of one of the knobs is commonly -y-J-y inch, and the 

 height is rather more. The sockets correspond in size. 



