444 PROCEEDINGS OF THE ACADEMY OF [J line, 



(5) Very large specimens are carried by the gulls to a height and 

 dropped on any near-by flat rock in the same way as the gulls treat 

 both sea urchins (Strongylocentrotus) and welks {Buccinum). 

 Only the largest Purpura will break and only the largest are collected 

 for this purpose. 



(6) Certain ones seem to be parasitized by a sporocyst, but this 

 was not investigated. 



(7) Accidents, such as a surf heavy enough to move the rocks on 

 which the animals live, may destroy others. In such habitats few 

 if any are found. The maximum size of these rocks may in exposed 

 places be boulders two to three feet in diameter, while in very shel- 

 tered places Thais will be found on pebbles two inches in diameter. 

 The limit is obviously determined by wave action. 



(8) Ice probably destroys others, as they are not found in places 

 where shore ice can accumulate. 



(a) In Maine they are not abundant in shallow coves that freeze 

 up solid in the winter time. 



(b) Their geographical distribution also seems to indicate that 

 sand beaches are a barrier, as Thais lapillus is not found west of 

 Montauk Point. On Long Island Sound the barrier has not been 

 determined. Both Balanus and Mytilus being found west of Stam- 

 ford, Conn., the most westerly recorded station for Thais, and the 

 shore is still rocky. In the north shore ice seemed to form the 

 barrier See map, fig. 3. 



Various adaptations are at once manifest on examining this 

 animal: (1) The strong shell, so strong that a drop from a height 

 will not always break it. (2) The strong foot, which makes dis- 

 lodgment from its substratum difficult. (3) Its habit of seeking 

 shelter under rocks and fucus. (4) The method of egg-laying, with 

 its protection of the young until they are able to care for themselves, 

 although if but two and a quarter per cent, of these laid hatch, those 

 two and a quarter per cent, are almost sure to hatch. 



Method. 



As each station was visited, from 200 to 500 shells were collected 

 when Thais was abundant. In some places they literally covered 

 every inch of the rock. In other places they were so rare that a 

 careful search of a quarter of a mile of beach would often disclose 

 but two or three. 



Each collection as made was placed in a bag made of mosquito 

 netting. While still alive the snails were sorted into the dozen or 



