170 MR. H. L. HAWKIXS ON THE 



of the various views held on that point follows the description of 

 the anterior ambulacrum of E. cordatuni. 



II. Technique. 



The test in ambulacrum III. of E. cordatum is exceedingly- 

 thin, even in large examples being only as thick as ordinary 

 paper. Viewed from tlie outside, the area between the apical 

 system and the anterior border of the internal fasciole is covered 

 with an almost uniform granulation, and no trace of sutures can 

 be distinguished in the adapical part. From the inside a clue 

 can be obtained as to the existence of complexity in the structure 

 of the area, but it seems a general rule among Echinoids that the 

 sutures shown on the inner surface of the test differ to some 

 degree in their disposition from those on the outside. After a 

 great number of experiments, I have perfected a method whereby, 

 as may be judged from PI. XXVI. fig. 2, the details of the 

 sutures, however minute, may be rendered clear. 



The specimen should be well dried, and may have been stoi'ed 

 in a collection for many years without lessening the efficacy of the 

 process. It should not, however, be macerated, and the bleached 

 specimens of E. cordatum often found on sand-dunes rarely prove 

 suitable material. 



When the specimen is quite dry and brittle, the test should be 

 split open along the ambitus, and all the sandy and organic con- 

 tents removed. Especial care is needed to remove all the dried- 

 up ampullae of the area to be examined. It is usually necessary 

 to moisten the specimen for this purpose. When all the radioles 

 and accessible organic matter have been brushed away from both 

 surfaces, the specimen should be thoroughly dried again. A 

 hurried drying at this stage (over a flame or near a fire) gives the 

 best results. 



The part to be examined is then saturated with a staining 

 solution. Of all the fluids that I have used, none give clearer 

 results than the aniline colours supplied as cheap ink at a penny 

 per bottle. The red, blue, and green colours are all equally 

 efiective, but I prefer to use the last-named, as being more 

 restful to the eyes, and admii-ably suited to the requirements of 

 photography. It is advisable to apply the stain with a fine 

 camel's-hair brush to both the inner and outer surfaces, and 

 the test should be quite saturated. 



When the area is again perfectly dry, the brush should be 

 dipped into strong hydrochloric acid, and drawn lightly once or 

 twice over the outer surface. Care must, of course, be taken to 

 wash off all the acid (from both surfaces) very quickly. The 

 etching with acid should be continued until the sutures stand 

 out clearly. The pi-ocess depends on the fact that a thin film 

 of organic matter exists between the plates, so that, when the 

 calcareous substance is partly etched away, the densely stained 

 tissue in the sutures remains standing' out in relief. 



