1IYALEA. 381 



the lateral spines is delicately linear ; both lips are briefly 

 and abruptly reflected. 



The upper and lower surfaces are of nearly equal con- 

 vexity, and are both of them, more frequently than other- 

 wise, devoid of well marked concentric strise. The former 

 projects semicircularly, but only slightly, beyond the 

 straightish anterior or labial edge of the latter, and is 

 adorned with a broad rounded central fold, which is ge- 

 nerally, if not always, subdivided in front into three smaller 

 ones, flanked on each side with a strong but narrower fold, 

 which leaves a flattish triangular space between it and the 

 termination of the lateral spine. Merely a slight fold 

 adjacent to each lateral spine is observable upon the lower 

 superficies. The posterior edges, previous to the com- 

 mencement of the caudal spine, which when perfect is at 

 least equal in length to the space from its origin to the 

 lateral aculeations, and tapers with extreme slowness, run 

 towards each other subrectilinearly and nearly at right 

 angles. The specimens we have described from, measure 

 at least five lines in length, and three and a half in breadth 

 from prickle to prickle. 



The circumstances under which this shell has been taken 

 are as follows. About the year 1820 a large mast floated 

 into Youghal, covered with Anatifa lavis, and perforated 

 by Teredines. From the water in which the sawn-off end 

 had been laved, Mr. Robert Ball of Dublin obtained a 

 Spirilla and a Hyalea ; the latter shell was still filled 

 with animal matter, although life was extinct in the Pter- 

 opod. From the roving habits of the genus, it is by no 

 means improbable that many more examples have at times 

 been wafted upon our shores, but perished through their 

 extreme fragility, before attracting observation. 



