508 Miss JOAN B. PROCTEH ON THE 



, the oblique connecting bridges between the costal plates, which 

 in turn underlie the vertebro- costal epidermal shield-sutures : 

 that is to say, the point where the rib becomes adherent. 



The capitular free portion of the rib absorbs in a peculiar wa,y. 

 At 71 mm. these free ribs still appear to be present when the 

 carapace of a spirit specimen is raised; when explored, however, 

 they prove to be chiefly composed of soft pliant material, like 

 ligament. At 81 mm. there is still a splinter-like rib within this 

 soft material, but this becomes more and more wasted by 

 absorption, until it is lost absolutely in the adult, or represented 

 by a tiny thorn -like splinter one or two mm. in length — the 

 extreme ca.pitulum of the original rib. Mr. Burne has very 

 kindly investigated the structure of this soft portion, excised 

 from a young specimen. Longitudinal sections show that with 



Text-figure 16. 



Microscopic section through capitulum of absorbing rib(X 100), in which ahnost 

 nothing but periosteum is left. (After a sketch by Mr. Burne.) Periosteal 

 fibres, p ; cartilage, c ; areas of calcification, h. 



the exception of two small areas of calcification in the capitulum 

 itself, no bone is left {vide text-fig. 16). These areas are situated 

 in a part of the original cartilage ; the rest of the section shows 

 nothing but parallel bundles of what appears to be connective 

 tissue. Near the edge of the sections there are, however, a row 

 of multi -nucleated cells which Mr. Burne considers to be osteo- 

 clasts. He says : " I presume what happens is that the bone 

 after being formed in the rib is absorbed by these osteoclasts, 

 and the periosteum — possibly thickened — is left, forming a simu- 

 lacrum of the rib." There is no doubt in my mind that this is the 

 right interpretation ; also that this process of absorption, carried 

 only to an early stage, is responsible for the excessive slender- 

 ness of this portion of the ribs in other species. 



The carapace. 



In a specimen 42 mm. in length, probably newly hatched or 

 only a few months old, the plates are commencing their develop- 

 ment, and are at a particularly interesting stage {vide text-fig. 17). 



The niichal is more or less fully formed within the marginal 

 ring, but does not extend beyond. The neurals, excepting the 

 third and fifth, consist of minute oval areas of yellowish bone- 

 granules towaids the anterior of each underlying vertebra. The 

 third and fifth are laterally dilated to some extent, the fifth being 

 almost in contact with the hea.d of the fifth costal plate. These 

 two neurals undei-lie the two middle vertebi-al shield-sutures. 

 The thin layer of coarse bone-granules which composes each 



